Re: [PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected by string ops
On Tue, Apr 22, 2014 at 09:04:45AM +0300, Nadav Amit wrote:
Gleb,
On 4/20/14, 12:26 PM, Gleb Natapov wrote:
On Fri, Apr 18, 2014 at 07:11:33AM +0300, Nadav Amit wrote:
When using address-size override prefix with string instructions in
long-mode,
ESI/EDI/ECX are zero extended if they are affected by the instruction
(incremented/decremented). Currently, the KVM emulator does not do so.
In addition, although it is not well-documented, when address override
prefix
is used with REP-string instruction, RCX high half is zeroed even if ECX was
zero on the first iteration. Therefore, the emulator should clear the upper
part of RCX in this case, as x86 CPUs do.
Signed-off-by: Nadav Amit na...@cs.technion.ac.il
---
:100644 100644 69e2636... a69ed67... M arch/x86/kvm/emulate.c
arch/x86/kvm/emulate.c |4
1 file changed, 4 insertions(+)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 69e2636..a69ed67 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -491,6 +491,8 @@ register_address_increment(struct x86_emulate_ctxt
*ctxt, unsigned long *reg, in
else
mask = ad_mask(ctxt);
masked_increment(reg, mask, inc);
+ if (ctxt-ad_bytes == 4)
+ *reg = 0x;
*reg=(u32)*reg; and you can do it inside else part.
register_address_increment() is used also by jmp_rel and loop instructions,
is this correct for both of those too? Probably yes.
It appears to be so.
Results of 32-bit operations are implicitly zero extended to 64-bit
values, and this appears to apply to all 32 bit operations,
including implicit ones. Therefore it seems to apply to all these
operations.
}
static void rsp_increment(struct x86_emulate_ctxt *ctxt, int inc)
@@ -4567,6 +4569,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
if (ctxt-rep_prefix (ctxt-d String)) {
/* All REP prefixes have the same first termination condition */
if (address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) == 0) {
+ if (ctxt-ad_bytes == 4)
+ *reg_write(ctxt, VCPU_REGS_RCX) = 0;
Does zero extension happens even if ECX was zero at the beginning on an
instruction or only during
ECX modification. If later it is already covered in
register_address_increment, no?
The observed behaviour of the Sandy-Bridge I use, is that even if
ECX is zero on the first iteration, the high half of RCX is zeroed.
Therefore, this is a different case, which was not covered in
register_address_increment. I agree it is totally undocumented.
Following your previous comment - I may have missed the case in
which loop instruction is executed with ECX = 0 while RCX != 0 and
the address size is 32 bit. I will test this case soon (yet, it is
lower on my priority list).
In 64-bit mode, the operand size for all near branches (CALL, RET, JCC,
JCXZ, JMP, and LOOP) is forced to 64 bits.
These instructions update the 64-bit RIP without the need for a REX
operand-size prefix.
The following aspects of near branches are controlled by the effective
operand size:
• Truncation of the size of the instruction pointer
...
In 64-bit mode, all of the above actions are forced to 64 bits
regardless of operand size prefixes (operand size
prefixes are silently ignored). However, the displacement field for
relative branches is still limited to 32 bits and the
address size for near branches is not forced in 64-bit mode.
Address sizes affect the size of RCX used for JCXZ and LOOP; they also
impact the address calculation for memory
indirect branches. Such addresses are 64 bits by default; but they can
be overridden to 32 bits by an address size
prefix.
So it seems your patch incorrectly handles rex call for example.
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Re: [PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected by string ops
On Wed, Apr 23, 2014 at 04:58:32PM -0300, Marcelo Tosatti wrote:
On Tue, Apr 22, 2014 at 09:04:45AM +0300, Nadav Amit wrote:
Gleb,
On 4/20/14, 12:26 PM, Gleb Natapov wrote:
On Fri, Apr 18, 2014 at 07:11:33AM +0300, Nadav Amit wrote:
When using address-size override prefix with string instructions in
long-mode,
ESI/EDI/ECX are zero extended if they are affected by the instruction
(incremented/decremented). Currently, the KVM emulator does not do so.
In addition, although it is not well-documented, when address override
prefix
is used with REP-string instruction, RCX high half is zeroed even if ECX
was
zero on the first iteration. Therefore, the emulator should clear the
upper
part of RCX in this case, as x86 CPUs do.
Signed-off-by: Nadav Amit na...@cs.technion.ac.il
---
:100644 100644 69e2636... a69ed67... March/x86/kvm/emulate.c
arch/x86/kvm/emulate.c |4
1 file changed, 4 insertions(+)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 69e2636..a69ed67 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -491,6 +491,8 @@ register_address_increment(struct x86_emulate_ctxt
*ctxt, unsigned long *reg, in
else
mask = ad_mask(ctxt);
masked_increment(reg, mask, inc);
+ if (ctxt-ad_bytes == 4)
+ *reg = 0x;
*reg=(u32)*reg; and you can do it inside else part.
register_address_increment() is used also by jmp_rel and loop instructions,
is this correct for both of those too? Probably yes.
It appears to be so.
Results of 32-bit operations are implicitly zero extended to 64-bit
values, and this appears to apply to all 32 bit operations,
including implicit ones. Therefore it seems to apply to all these
operations.
}
static void rsp_increment(struct x86_emulate_ctxt *ctxt, int inc)
@@ -4567,6 +4569,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
if (ctxt-rep_prefix (ctxt-d String)) {
/* All REP prefixes have the same first termination
condition */
if (address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX))
== 0) {
+ if (ctxt-ad_bytes == 4)
+ *reg_write(ctxt, VCPU_REGS_RCX) = 0;
Does zero extension happens even if ECX was zero at the beginning on an
instruction or only during
ECX modification. If later it is already covered in
register_address_increment, no?
The observed behaviour of the Sandy-Bridge I use, is that even if
ECX is zero on the first iteration, the high half of RCX is zeroed.
Therefore, this is a different case, which was not covered in
register_address_increment. I agree it is totally undocumented.
Following your previous comment - I may have missed the case in
which loop instruction is executed with ECX = 0 while RCX != 0 and
the address size is 32 bit. I will test this case soon (yet, it is
lower on my priority list).
In 64-bit mode, the operand size for all near branches (CALL, RET, JCC,
JCXZ, JMP, and LOOP) is forced to 64 bits.
These instructions update the 64-bit RIP without the need for a REX
operand-size prefix.
The following aspects of near branches are controlled by the effective
operand size:
• Truncation of the size of the instruction pointer
...
In 64-bit mode, all of the above actions are forced to 64 bits
regardless of operand size prefixes (operand size
prefixes are silently ignored). However, the displacement field for
relative branches is still limited to 32 bits and the
address size for near branches is not forced in 64-bit mode.
Address sizes affect the size of RCX used for JCXZ and LOOP; they also
impact the address calculation for memory
indirect branches. Such addresses are 64 bits by default; but they can
be overridden to 32 bits by an address size
prefix.
So it seems your patch incorrectly handles rex call for example.
Err, operand size is forced to 64-bits, not address size.
The following aspects of near branches are controlled by the effective
operand size:
• Truncation of the size of the instruction pointer
Still, 67h call should not truncate EIP (which your patch does).
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Re: [PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected by string ops
On 4/23/14, 11:11 PM, Marcelo Tosatti wrote:
On Wed, Apr 23, 2014 at 04:58:32PM -0300, Marcelo Tosatti wrote:
On Tue, Apr 22, 2014 at 09:04:45AM +0300, Nadav Amit wrote:
Gleb,
On 4/20/14, 12:26 PM, Gleb Natapov wrote:
On Fri, Apr 18, 2014 at 07:11:33AM +0300, Nadav Amit wrote:
When using address-size override prefix with string instructions in long-mode,
ESI/EDI/ECX are zero extended if they are affected by the instruction
(incremented/decremented). Currently, the KVM emulator does not do so.
In addition, although it is not well-documented, when address override prefix
is used with REP-string instruction, RCX high half is zeroed even if ECX was
zero on the first iteration. Therefore, the emulator should clear the upper
part of RCX in this case, as x86 CPUs do.
Signed-off-by: Nadav Amit na...@cs.technion.ac.il
---
:100644 100644 69e2636... a69ed67... M arch/x86/kvm/emulate.c
arch/x86/kvm/emulate.c |4
1 file changed, 4 insertions(+)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 69e2636..a69ed67 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -491,6 +491,8 @@ register_address_increment(struct x86_emulate_ctxt *ctxt,
unsigned long *reg, in
else
mask = ad_mask(ctxt);
masked_increment(reg, mask, inc);
+ if (ctxt-ad_bytes == 4)
+ *reg = 0x;
*reg=(u32)*reg; and you can do it inside else part.
register_address_increment() is used also by jmp_rel and loop instructions,
is this correct for both of those too? Probably yes.
It appears to be so.
Results of 32-bit operations are implicitly zero extended to 64-bit
values, and this appears to apply to all 32 bit operations,
including implicit ones. Therefore it seems to apply to all these
operations.
}
static void rsp_increment(struct x86_emulate_ctxt *ctxt, int inc)
@@ -4567,6 +4569,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
if (ctxt-rep_prefix (ctxt-d String)) {
/* All REP prefixes have the same first termination condition */
if (address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) == 0) {
+ if (ctxt-ad_bytes == 4)
+ *reg_write(ctxt, VCPU_REGS_RCX) = 0;
Does zero extension happens even if ECX was zero at the beginning on an
instruction or only during
ECX modification. If later it is already covered in register_address_increment,
no?
The observed behaviour of the Sandy-Bridge I use, is that even if
ECX is zero on the first iteration, the high half of RCX is zeroed.
Therefore, this is a different case, which was not covered in
register_address_increment. I agree it is totally undocumented.
Following your previous comment - I may have missed the case in
which loop instruction is executed with ECX = 0 while RCX != 0 and
the address size is 32 bit. I will test this case soon (yet, it is
lower on my priority list).
In 64-bit mode, the operand size for all near branches (CALL, RET, JCC,
JCXZ, JMP, and LOOP) is forced to 64 bits.
These instructions update the 64-bit RIP without the need for a REX
operand-size prefix.
The following aspects of near branches are controlled by the effective
operand size:
• Truncation of the size of the instruction pointer
...
In 64-bit mode, all of the above actions are forced to 64 bits
regardless of operand size prefixes (operand size
prefixes are silently ignored). However, the displacement field for
relative branches is still limited to 32 bits and the
address size for near branches is not forced in 64-bit mode.
Address sizes affect the size of RCX used for JCXZ and LOOP; they also
impact the address calculation for memory
indirect branches. Such addresses are 64 bits by default; but they can
be overridden to 32 bits by an address size
prefix.
So it seems your patch incorrectly handles rex call for example.
Err, operand size is forced to 64-bits, not address size.
The following aspects of near branches are controlled by the effective
operand size:
• Truncation of the size of the instruction pointer
Still, 67h call should not truncate EIP (which your patch does).
Yes, I missed it.
But if I am not mistaken again, it means that the existing
implementation of jmp_rel is broken as well when address-size override
prefix is used. In this case, as I see it, the existing masking would
cause the carry from the add operation to the lower half of the rip not
to be added to the rip higher half.
I guess another patch is needed for that as well.
Nadav
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Re: [PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected by string ops
On 04/23/2014 01:53 PM, Nadav Amit wrote: Err, operand size is forced to 64-bits, not address size. The following aspects of near branches are controlled by the effective operand size: • Truncation of the size of the instruction pointer Still, 67h call should not truncate EIP (which your patch does). Yes, I missed it. But if I am not mistaken again, it means that the existing implementation of jmp_rel is broken as well when address-size override prefix is used. In this case, as I see it, the existing masking would cause the carry from the add operation to the lower half of the rip not to be added to the rip higher half. I guess another patch is needed for that as well. Yes, on x86 JMP really should be thought of as MOV ...,IP/EIP/RIP. On some other architectures, e.g. m68k, JMP acts as if it was LEA ...,PC, which causes some serious confusion for people familiar with that model. However, on x86 considering JMP as a MOV to the IP register really is very consistent and will give you the right mental model. -hpa -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected by string ops
Gleb,
On 4/20/14, 12:26 PM, Gleb Natapov wrote:
On Fri, Apr 18, 2014 at 07:11:33AM +0300, Nadav Amit wrote:
When using address-size override prefix with string instructions in long-mode,
ESI/EDI/ECX are zero extended if they are affected by the instruction
(incremented/decremented). Currently, the KVM emulator does not do so.
In addition, although it is not well-documented, when address override prefix
is used with REP-string instruction, RCX high half is zeroed even if ECX was
zero on the first iteration. Therefore, the emulator should clear the upper
part of RCX in this case, as x86 CPUs do.
Signed-off-by: Nadav Amit na...@cs.technion.ac.il
---
:100644 100644 69e2636... a69ed67... M arch/x86/kvm/emulate.c
arch/x86/kvm/emulate.c |4
1 file changed, 4 insertions(+)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 69e2636..a69ed67 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -491,6 +491,8 @@ register_address_increment(struct x86_emulate_ctxt *ctxt,
unsigned long *reg, in
else
mask = ad_mask(ctxt);
masked_increment(reg, mask, inc);
+ if (ctxt-ad_bytes == 4)
+ *reg = 0x;
*reg=(u32)*reg; and you can do it inside else part.
register_address_increment() is used also by jmp_rel and loop instructions,
is this correct for both of those too? Probably yes.
It appears to be so.
Results of 32-bit operations are implicitly zero extended to 64-bit
values, and this appears to apply to all 32 bit operations, including
implicit ones. Therefore it seems to apply to all these operations.
}
static void rsp_increment(struct x86_emulate_ctxt *ctxt, int inc)
@@ -4567,6 +4569,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
if (ctxt-rep_prefix (ctxt-d String)) {
/* All REP prefixes have the same first termination condition */
if (address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) == 0) {
+ if (ctxt-ad_bytes == 4)
+ *reg_write(ctxt, VCPU_REGS_RCX) = 0;
Does zero extension happens even if ECX was zero at the beginning on an
instruction or only during
ECX modification. If later it is already covered in register_address_increment,
no?
The observed behaviour of the Sandy-Bridge I use, is that even if ECX is
zero on the first iteration, the high half of RCX is zeroed. Therefore,
this is a different case, which was not covered in
register_address_increment. I agree it is totally undocumented.
Following your previous comment - I may have missed the case in which
loop instruction is executed with ECX = 0 while RCX != 0 and the address
size is 32 bit. I will test this case soon (yet, it is lower on my
priority list).
Nadav
--
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the body of a message to majord...@vger.kernel.org
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Re: [PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected by string ops
On Fri, Apr 18, 2014 at 07:11:33AM +0300, Nadav Amit wrote:
When using address-size override prefix with string instructions in long-mode,
ESI/EDI/ECX are zero extended if they are affected by the instruction
(incremented/decremented). Currently, the KVM emulator does not do so.
In addition, although it is not well-documented, when address override prefix
is used with REP-string instruction, RCX high half is zeroed even if ECX was
zero on the first iteration. Therefore, the emulator should clear the upper
part of RCX in this case, as x86 CPUs do.
Signed-off-by: Nadav Amit na...@cs.technion.ac.il
---
:100644 100644 69e2636... a69ed67... March/x86/kvm/emulate.c
arch/x86/kvm/emulate.c |4
1 file changed, 4 insertions(+)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 69e2636..a69ed67 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -491,6 +491,8 @@ register_address_increment(struct x86_emulate_ctxt *ctxt,
unsigned long *reg, in
else
mask = ad_mask(ctxt);
masked_increment(reg, mask, inc);
+ if (ctxt-ad_bytes == 4)
+ *reg = 0x;
*reg=(u32)*reg; and you can do it inside else part.
register_address_increment() is used also by jmp_rel and loop instructions,
is this correct for both of those too? Probably yes.
}
static void rsp_increment(struct x86_emulate_ctxt *ctxt, int inc)
@@ -4567,6 +4569,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
if (ctxt-rep_prefix (ctxt-d String)) {
/* All REP prefixes have the same first termination condition */
if (address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) == 0) {
+ if (ctxt-ad_bytes == 4)
+ *reg_write(ctxt, VCPU_REGS_RCX) = 0;
Does zero extension happens even if ECX was zero at the beginning on an
instruction or only during
ECX modification. If later it is already covered in register_address_increment,
no?
ctxt-eip = ctxt-_eip;
goto done;
}
--
1.7.10.4
--
Gleb.
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