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

[Binbin Wu]

On 5/15/2026 8:55 PM, Sean Christopherson wrote:
> 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.

OK, I can do it.

> 
>> 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);
>>
>