On 18.07.2022 22:50, Andrew Cooper wrote:
> --- a/xen/arch/x86/hvm/vmx/entry.S
> +++ b/xen/arch/x86/hvm/vmx/entry.S
> @@ -33,13 +33,12 @@ ENTRY(vmx_asm_vmexit_handler)
> movb $1,VCPU_vmx_launched(%rbx)
> mov %rax,VCPU_hvm_guest_cr2(%rbx)
>
> - /* SPEC_CTRL_ENTRY_FROM_VMX Req: b=curr %rsp=regs/cpuinfo, Clob:
> acd */
> + /* SPEC_CTRL_ENTRY_FROM_VMX Req: %rsp=regs/cpuinfo, %rdx=0, Clob:
> acd */
> ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
Leaving %rdx documented as clobbered here is misleading - the scope of
the comment is likely meant to extend to ...
> .macro restore_spec_ctrl
> mov $MSR_SPEC_CTRL, %ecx
> movzbl CPUINFO_xen_spec_ctrl(%rsp), %eax
> - xor %edx, %edx
> wrmsr
> .endm
> ALTERNATIVE "", restore_spec_ctrl, X86_FEATURE_SC_MSR_HVM
> @@ -49,7 +48,6 @@ ENTRY(vmx_asm_vmexit_handler)
> .macro restore_lbr
> mov $IA32_DEBUGCTLMSR_LBR, %eax
> mov $MSR_IA32_DEBUGCTLMSR, %ecx
> - xor %edx, %edx
> wrmsr
> .endm
> ALTERNATIVE "", restore_lbr, X86_FEATURE_XEN_LBR
... here, but that's not necessarily what a reader might gain. Plus
with the change the register isn't clobbered anymore.
> --- a/xen/arch/x86/include/asm/spec_ctrl_asm.h
> +++ b/xen/arch/x86/include/asm/spec_ctrl_asm.h
> @@ -176,7 +176,7 @@
> .macro DO_SPEC_CTRL_ENTRY maybexen:req
> /*
> * Requires %rsp=regs (also cpuinfo if !maybexen)
> - * Requires %r14=stack_end (if maybexen)
> + * Requires %r14=stack_end (if maybexen), %rdx=0
> * Clobbers %rax, %rcx, %rdx
> *
> * PV guests can't update MSR_SPEC_CTRL behind Xen's back, so no need to read
> @@ -184,7 +184,6 @@
> * while entries from Xen must leave shadowing in its current state.
> */
> mov $MSR_SPEC_CTRL, %ecx
> - xor %edx, %edx
>
> /*
> * Clear SPEC_CTRL shadowing *before* loading Xen's value. If entering
This is used in SPEC_CTRL_ENTRY_FROM_{INTR,PV} after
DO_SPEC_CTRL_COND_IBPB, which documents %rdx as clobbered. Since it
doesn't actually clobber the register, it's the documentation line
which needs updating along with making this code change.
With the respective adjustments
Reviewed-by: Jan Beulich <jbeul...@suse.com>
Jan
