Re: [PATCH 03/11] KVM: x86/xen: Don't truncate RAX when handling hypercall from protected guest

[Sean Christopherson <seanjc@google.com>]

On Mon, Apr 13, 2026, Binbin Wu wrote:
> 
> 
> On 4/10/2026 7:56 AM, Sean Christopherson wrote:
> > Don't truncate RAX when handling a Xen hypercall for a guest with protected
> > state, 
> 
> It sounds like KVM supports Xen hypercalls from a guest with protected state

"supports" is a strong word.  It's theoretically possible.  I highly doubt anyone
has evern actually tried to smush the two together.

> normally, but it seems that a warning would still be triggered by
> kvm_rax_read() for checking whether it's a Hyper-V hypercall even after the
> whole patch set when the userspace enables KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL
> for the guest.

Ooh, fun.

> > 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@google.com>
> > ---
> >  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) &&
> 
> Should this function call be replaced with kvm_rax_read_raw() in patch 7/11
> if KVM allows the Xen hypercalls from a guest with protected state to avoid
> triggering the warning?

Yeah, that does seem like the correct behavior.

> >  	    kvm_hv_hypercall_enabled(vcpu))
> >  		return kvm_hv_hypercall(vcpu);
> >  
> >  	longmode = is_64_bit_hypercall(vcpu);
> >  	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);
>