On Fri, Jan 18, 2019 at 01:20:11PM -0800, Sean Christopherson wrote: > In the vCPU-run asm blob, the guest's RCX is temporarily saved onto the > stack after VM-Exit as the exit flow must first load a register with a > pointer to the vCPU's save area in order to save the guest's registers. > RCX is arbitrarily designated as the scratch register. > > Since the stack usage is to (1)save host, (2)save guest, (3)load host > and (4)load guest, the code can't conform to the stack's natural FIFO > semantics, i.e. it can't simply do PUSH/POP. Regardless of whether it > is done for the host's value or guest's value, at some point the code > needs to access the stack using a non-traditional method, e.g. MOV > instead of POP. vCPU-run opts to create a placeholder on the stack for > guest's RCX (by adjusting RSP) and saves RCX to its place immediately > after VM-Exit (via MOV). > > In other words, the purpose of the first 'PUSH RCX' at the start of > the vCPU-run asm blob is to adjust RSP down, i.e. there's no need to > actually access memory. Use 'SUB $wordsize, RSP' instead of 'PUSH RCX' > to make it more obvious that the intent is simply to create a gap on > the stack for the guest's RCX. > > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> Thank you! > --- > arch/x86/kvm/vmx/vmx.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index 23e5fa58751a..adf59fd23a6c 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -6377,7 +6377,7 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx) > asm( > /* Store host registers */ > "push %%" _ASM_DX "; push %%" _ASM_BP ";" > - "push %%" _ASM_CX " \n\t" /* placeholder for guest rcx */ > + "sub $%c[wordsize], %%" _ASM_SP "\n\t" /* placeholder for guest RCX */ > "push %%" _ASM_CX " \n\t" > "sub $%c[wordsize], %%" _ASM_SP "\n\t" /* temporarily adjust RSP for CALL */ > "cmp %%" _ASM_SP ", %c[host_rsp](%%" _ASM_CX ") \n\t" > -- > 2.20.1 >
