Re: [PATCH v2 03/15] KVM: x86/xen: Don't truncate RAX when handling hypercall from protected guest

[Sean Christopherson]

On Fri, May 15, 2026, Binbin Wu wrote:
> 
> 
> On 5/15/2026 5:53 AM, Sean Christopherson wrote:
> > Don't truncate RAX when handling a Xen hypercall for a guest with protected
> > state, as KVM's ABI is to assume the guest is in 64-bit for such cases
> > (the guest leaving garbage in 63:32 after a transition to 32-bit mode is
> > far less likely than 63:32 being necessary to complete the hypercall).
> > 
> > Fixes: b5aead0064f3 ("KVM: x86: Assume a 64-bit hypercall for guests with protected state")
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> 
> The patch looks good to me, but one question below.
> 
> > ---
> >  arch/x86/kvm/xen.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> > index 6d9be74bb673..895095dc684e 100644
> > --- a/arch/x86/kvm/xen.c
> > +++ b/arch/x86/kvm/xen.c
> > @@ -1678,15 +1678,14 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
> >  	bool handled = false;
> >  	u8 cpl;
> >  
> > -	input = (u64)kvm_register_read(vcpu, VCPU_REGS_RAX);
> > -
> >  	/* Hyper-V hypercalls get bit 31 set in EAX */
> > -	if ((input & 0x80000000) &&
> > +	if ((kvm_rax_read(vcpu) & 0x80000000) &&
> >  	    kvm_hv_hypercall_enabled(vcpu))
> >  		return kvm_hv_hypercall(vcpu);
> >  
> >  	longmode = is_64_bit_hypercall(vcpu);
> 
> Is the variable name misleading?

It most definitely is.  However, @longmode is passed around quite a few locations
in xen.c, and so I don't want to opportunistically fix this one variable.  Though
I'm definitely not opposed to a separate patch to rename them all to is_64bit or
something.

> If the vcpu is in compatible mode (when guest state is not protected),
> it's in long mode, but the code goes to !longmode path.
> 
> >  	if (!longmode) {
> > +		input = (u32)kvm_rax_read(vcpu);
> >  		params[0] = (u32)kvm_rbx_read(vcpu);
> >  		params[1] = (u32)kvm_rcx_read(vcpu);
> >  		params[2] = (u32)kvm_rdx_read(vcpu);
> > @@ -1696,6 +1695,7 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
> >  	}
> >  	else {
> >  #ifdef CONFIG_X86_64
> > +		input = (u64)kvm_rax_read(vcpu);
> >  		params[0] = (u64)kvm_rdi_read(vcpu);
> >  		params[1] = (u64)kvm_rsi_read(vcpu);
> >  		params[2] = (u64)kvm_rdx_read(vcpu);
>