Re: [PATCH v2 3/5] KVM: VMX: Add error handling to VMREAD helper
On 28/07/19 21:36, Josh Poimboeuf wrote:
> On Fri, Jul 19, 2019 at 01:41:08PM -0700, Sean Christopherson wrote:
>> @@ -68,8 +67,22 @@ static __always_inline unsigned long
>> __vmcs_readl(unsigned long field)
>> {
>> unsigned long value;
>>
>> -asm volatile (__ex_clear("vmread %1, %0", "%k0")
>> - : "=r"(value) : "r"(field));
>> +asm volatile("1: vmread %2, %1\n\t"
>> + ".byte 0x3e\n\t" /* branch taken hint */
>> + "ja 3f\n\t"
>> + "mov %2, %%" _ASM_ARG1 "\n\t"
>> + "xor %%" _ASM_ARG2 ", %%" _ASM_ARG2 "\n\t"
>> + "2: call vmread_error\n\t"
>> + "xor %k1, %k1\n\t"
>> + "3:\n\t"
>> +
>> + ".pushsection .fixup, \"ax\"\n\t"
>> + "4: mov %2, %%" _ASM_ARG1 "\n\t"
>> + "mov $1, %%" _ASM_ARG2 "\n\t"
>> + "jmp 2b\n\t"
>> + ".popsection\n\t"
>> + _ASM_EXTABLE(1b, 4b)
>> + : ASM_CALL_CONSTRAINT, "=r"(value) : "r"(field) : "cc");
>
> Was there a reason you didn't do the asm goto thing here like you did
> for the previous patch? That seemed cleaner, and needs less asm.
It's because asm goto doesn't support outputs.
Paolo
> I think the branch hints aren't needed -- they're ignored on modern
> processors. Ditto for the previous patch.
>
> Also please use named asm operands whereever you can, like "%[field]"
> instead of "%2". It helps a lot with readability.
>
Re: [PATCH v2 3/5] KVM: VMX: Add error handling to VMREAD helper
On Fri, Jul 19, 2019 at 01:41:08PM -0700, Sean Christopherson wrote:
> @@ -68,8 +67,22 @@ static __always_inline unsigned long __vmcs_readl(unsigned
> long field)
> {
> unsigned long value;
>
> - asm volatile (__ex_clear("vmread %1, %0", "%k0")
> - : "=r"(value) : "r"(field));
> + asm volatile("1: vmread %2, %1\n\t"
> + ".byte 0x3e\n\t" /* branch taken hint */
> + "ja 3f\n\t"
> + "mov %2, %%" _ASM_ARG1 "\n\t"
> + "xor %%" _ASM_ARG2 ", %%" _ASM_ARG2 "\n\t"
> + "2: call vmread_error\n\t"
> + "xor %k1, %k1\n\t"
> + "3:\n\t"
> +
> + ".pushsection .fixup, \"ax\"\n\t"
> + "4: mov %2, %%" _ASM_ARG1 "\n\t"
> + "mov $1, %%" _ASM_ARG2 "\n\t"
> + "jmp 2b\n\t"
> + ".popsection\n\t"
> + _ASM_EXTABLE(1b, 4b)
> + : ASM_CALL_CONSTRAINT, "=r"(value) : "r"(field) : "cc");
Was there a reason you didn't do the asm goto thing here like you did
for the previous patch? That seemed cleaner, and needs less asm.
I think the branch hints aren't needed -- they're ignored on modern
processors. Ditto for the previous patch.
Also please use named asm operands whereever you can, like "%[field]"
instead of "%2". It helps a lot with readability.
--
Josh
[PATCH v2 3/5] KVM: VMX: Add error handling to VMREAD helper
Now that VMREAD flows require a taken branch, courtesy of commit
3901336ed9887 ("x86/kvm: Don't call kvm_spurious_fault() from .fixup")
bite the bullet and add full error handling to VMREAD, i.e. replace the
JMP added by __ex()/kvm_handle_fault_on_reboot() with a hinted Jcc.
To minimize the code footprint, add a helper function, vmread_error(),
to handle both faults and failures so that the inline flow has a single
CALL.
Acked-by: Paolo Bonzini
Cc: Josh Poimboeuf
Signed-off-by: Sean Christopherson
---
arch/x86/kvm/vmx/ops.h | 21 +
arch/x86/kvm/vmx/vmx.c | 8
2 files changed, 25 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kvm/vmx/ops.h b/arch/x86/kvm/vmx/ops.h
index 79e25d49d4d9..45eaedee2ac0 100644
--- a/arch/x86/kvm/vmx/ops.h
+++ b/arch/x86/kvm/vmx/ops.h
@@ -11,9 +11,8 @@
#include "vmcs.h"
#define __ex(x) __kvm_handle_fault_on_reboot(x)
-#define __ex_clear(x, reg) \
- kvm_handle_fault_on_reboot(x, "xor " reg ", " reg)
+asmlinkage void vmread_error(unsigned long field, bool fault);
void vmwrite_error(unsigned long field, unsigned long value);
void vmclear_error(struct vmcs *vmcs, u64 phys_addr);
void vmptrld_error(struct vmcs *vmcs, u64 phys_addr);
@@ -68,8 +67,22 @@ static __always_inline unsigned long __vmcs_readl(unsigned
long field)
{
unsigned long value;
- asm volatile (__ex_clear("vmread %1, %0", "%k0")
- : "=r"(value) : "r"(field));
+ asm volatile("1: vmread %2, %1\n\t"
+".byte 0x3e\n\t" /* branch taken hint */
+"ja 3f\n\t"
+"mov %2, %%" _ASM_ARG1 "\n\t"
+"xor %%" _ASM_ARG2 ", %%" _ASM_ARG2 "\n\t"
+"2: call vmread_error\n\t"
+"xor %k1, %k1\n\t"
+"3:\n\t"
+
+".pushsection .fixup, \"ax\"\n\t"
+"4: mov %2, %%" _ASM_ARG1 "\n\t"
+"mov $1, %%" _ASM_ARG2 "\n\t"
+"jmp 2b\n\t"
+".popsection\n\t"
+_ASM_EXTABLE(1b, 4b)
+: ASM_CALL_CONSTRAINT, "=r"(value) : "r"(field) : "cc");
return value;
}
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 46689019ebf7..9acf3d6395d3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -349,6 +349,14 @@ do { \
pr_warn_ratelimited(fmt); \
} while (0)
+asmlinkage void vmread_error(unsigned long field, bool fault)
+{
+ if (fault)
+ kvm_spurious_fault();
+ else
+ vmx_insn_failed("kvm: vmread failed: field=%lx\n", field);
+}
+
noinline void vmwrite_error(unsigned long field, unsigned long value)
{
vmx_insn_failed("kvm: vmwrite failed: field=%lx val=%lx err=%d\n",
--
2.22.0
