Re: [PATCH 07/11] KVM: x86: Add mode-aware versions of kvm__{read,write}() helpers

Sean Christopherson
2026-04-14T15:42:37Z

On Tue, Apr 14, 2026, Kai Huang wrote:
>
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -757,7 +757,7 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm)
> >
> > svm->vcpu.arch.cr2 = save->cr2;
> >
> > - kvm_rax_write(vcpu, save->rax);
> > + kvm_rax_write_raw(vcpu, save->rax);
> > kvm_rsp_write(vcpu, save->rsp);
> > kvm_rip_write(vcpu, save->rip);
> >
> > @@ -1238,7 +1238,7 @@ static int nested_svm_vmexit_update_vmcb12(struct kvm_vcpu *vcpu)
> > vmcb12->save.rflags = kvm_get_rflags(vcpu);
> > vmcb12->save.rip = kvm_rip_read(vcpu);
> > vmcb12->save.rsp = kvm_rsp_read(vcpu);
> > - vmcb12->save.rax = kvm_rax_read(vcpu);
> > + vmcb12->save.rax = kvm_rax_read_raw(vcpu);
>
> Not sure whether it matters, I think there's an inconsistency here:
>
> The "rax" one has "raw" postfix, but "rsp" doesn't, despite in practice it
> is also a "raw" operation. Ditto for "rip", although it will be moved out
> of the "regs[]" GPR array.

Oh, there's very much an inconsistency. RIP probably "fine", as it should be
impossible to get a 64-bit RIP into the CPU when it's not in 64-bit mode. RSP
is likely not "fine", i.e. should probably use a "raw" version.

But most importantly, for this patch, I want to avoid introducing functional
changes, which means using the "raw" variant to read RAX.

> But maybe they are different?
>
> [...]
>
> > case EXIT_REASON_MSR_WRITE:
> > - kvm_rcx_write(vcpu, tdx->vp_enter_args.r12);
> > - kvm_rax_write(vcpu, tdx->vp_enter_args.r13 & -1u);
> > - kvm_rdx_write(vcpu, tdx->vp_enter_args.r13 >> 32);
> > + kvm_ecx_write(vcpu, tdx->vp_enter_args.r12);
> > + kvm_eax_write(vcpu, tdx->vp_enter_args.r13 & -1u);
>
> Nit: the "& -1u" isn't needed anymore with using kvm_eax_write(), but maybe
> we should just focus on replacing the functions in this patch but leave
> cleanup in the future.

Gah, good eyeballs. I intended to drop it here.

> [...]
>
>
> > @@ -12184,23 +12185,23 @@ static void __set_regs(struct kvm_vcpu *vcpu, struct kvm_regs *regs)
> > vcpu->arch.emulate_regs_need_sync_from_vcpu = true;
> > vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
> >
> > - kvm_rax_write(vcpu, regs->rax);
> > - kvm_rbx_write(vcpu, regs->rbx);
> > - kvm_rcx_write(vcpu, regs->rcx);
> > - kvm_rdx_write(vcpu, regs->rdx);
> > - kvm_rsi_write(vcpu, regs->rsi);
> > - kvm_rdi_write(vcpu, regs->rdi);
> > + kvm_rax_write_raw(vcpu, regs->rax);
> > + kvm_rbx_write_raw(vcpu, regs->rbx);
> > + kvm_rcx_write_raw(vcpu, regs->rcx);
> > + kvm_rdx_write_raw(vcpu, regs->rdx);
> > + kvm_rsi_write_raw(vcpu, regs->rsi);
> > + kvm_rdi_write_raw(vcpu, regs->rdi);
> > kvm_rsp_write(vcpu, regs->rsp);
> > - kvm_rbp_write(vcpu, regs->rbp);
> > + kvm_rbp_write_raw(vcpu, regs->rbp);
> >
>
> Ditto, the "rsp" one stands out. :-)

Yeah, same thing as above. I don't think the currently code is 100% correct, but
in practice it probably doesn't matter.

If we want to clean up RSP handling, it should definitely be done in a separate
patch (or patches, plural). But I'm hesitant to even try, especially for this
path since it's very much part of KVM's ABI. I.e. if ain't broke, don't fix it.

>
> [...]
>
> > +static __always_inline void kvm_e##lname##_write(struct kvm_vcpu *vcpu, u32 val) \
>
> It seems tab is used before the 'u32 val'. Seems no need.