Re: [PATCH 4/8] KVM: SVM: move guest vmsave/vmload to assembly

On Mon, Nov 07, 2022 at 09:54:32AM -0500, Paolo Bonzini wrote:

@@ -56,6 +59,16 @@ SYM_FUNC_START(__svm_vcpu_run) /* Move @svm to RDI. */ mov %_ASM_ARG2, %_ASM_DI

• /*

• * Use a single vmcb (vmcb01 because it's always valid) for
  

• * context switching guest state via VMLOAD/VMSAVE, that way
  

• * the state doesn't need to be copied between vmcb01 and
  

• * vmcb02 when switching vmcbs for nested virtualization.
  

• */
  

• mov SVM_vmcb01_pa(%_ASM_DI), %_ASM_AX

+1: vmload %_ASM_AX +2:

• /* "POP" @vmcb to RAX. */ pop %_ASM_AX

@@ -80,16 +93,11 @@ SYM_FUNC_START(__svm_vcpu_run) /* Enter guest mode */ sti +3: vmrun %_ASM_AX +4:

• cli

• /* Pop @svm to RAX while it's the only available register. */ pop %_ASM_AX

/* Save all guest registers. */

So Andrew noted that once the vmload has executed any exception taken (say at 3) will crash and burn because %gs is scribbled.

Might be good to make a record of this in the code so it can be cleaned up some day.

@@ -159,11 +179,19 @@ SYM_FUNC_START(__svm_vcpu_run) pop %_ASM_BP RET +10: cmpb $0, kvm_rebooting jne 2b ud2 +30: cmpb $0, kvm_rebooting

• jne 4b
• ud2

+50: cmpb $0, kvm_rebooting

• jne 6b
• ud2

• _ASM_EXTABLE(1b, 10b)
• _ASM_EXTABLE(3b, 30b)
• _ASM_EXTABLE(5b, 50b)