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KVM/VMX: Avoid stack engine synchronization uop in __vmx_vcpu_run

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Avoid instructions with explicit uses of the stack pointer between
instructions that implicitly refer to it. The sequence of
POP %reg; ADD $x, %RSP; POP %reg forces emission of synchronization
uop to synchronize the value of the stack pointer in the stack engine
and the out-of-order core.

Using POP with the dummy register instead of ADD $x, %RSP results in a
smaller code size and faster code.

The patch also fixes the reference to the wrong register in the
nearby comment.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20220816211010.25693-1-ubizjak@gmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Uros Bizjak 2022-08-16 23:10:10 +02:00 committed by Paolo Bonzini
parent 5a2a961be2
commit afe30b59d3
1 changed files with 4 additions and 5 deletions
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9
arch/x86/kvm/vmx/vmenter.S
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@ -189,13 +189,16 @@ SYM_INNER_LABEL(vmx_vmexit, SYM_L_GLOBAL)
xor %ebx, %ebx
.Lclear_regs:
/* Discard @regs. The register is irrelevant, it just can't be RBX. */
pop %_ASM_AX
/*
* Clear all general purpose registers except RSP and RBX to prevent
* speculative use of the guest's values, even those that are reloaded
* via the stack. In theory, an L1 cache miss when restoring registers
* could lead to speculative execution with the guest's values.
* Zeroing XORs are dirt cheap, i.e. the extra paranoia is essentially
* free. RSP and RAX are exempt as RSP is restored by hardware during
* free. RSP and RBX are exempt as RSP is restored by hardware during
* VM-Exit and RBX is explicitly loaded with 0 or 1 to hold the return
* value.
*/
@ -216,9 +219,6 @@ SYM_INNER_LABEL(vmx_vmexit, SYM_L_GLOBAL)
xor %r15d, %r15d
#endif
/* "POP" @regs. */
add $WORD_SIZE, %_ASM_SP
/*
* IMPORTANT: RSB filling and SPEC_CTRL handling must be done before
* the first unbalanced RET after vmexit!
@ -234,7 +234,6 @@ SYM_INNER_LABEL(vmx_vmexit, SYM_L_GLOBAL)
FILL_RETURN_BUFFER %_ASM_CX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT,\
X86_FEATURE_RSB_VMEXIT_LITE
pop %_ASM_ARG2 /* @flags */
pop %_ASM_ARG1 /* @vmx */