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

[Sean Christopherson <seanjc@google.com>]

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.