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

[Binbin Wu]

On 5/18/2026 5:50 PM, David Woodhouse wrote:
> On Mon, 2026-05-18 at 17:43 +0800, Binbin Wu wrote:
>>
>>
>> On 5/18/2026 3:15 PM, David Woodhouse wrote:
>>> On Mon, 2026-05-18 at 10:19 +0800, Binbin Wu wrote:
>>>>  
>>>>>>>   	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.
>>>
>>> This one (as shown above) is clearly indicating whether this particular
>>> vCPU is in 64-bit mode for this particular hypercall. Changing that to
>>> is_64bit makes sense.
>>>
>>> However, there is a separate overall mode for the VM, which is stored
>>> in 'kvm->arch.xen.long_mode' and accessed by userspace using the
>>> KVM_XEN_ATTR_TYPE_LONG_MODE attribute. It affects the datatypes used by
>>> shared memory data structures, and is also latched by the kernel when
>>> the guest writes the MSR for the hypercall page. That one should
>>> probably keep its name.
>>
>> For this one, I think the current KVM code is consistent.
>> The format is determined by EFER.LMA, whether the guest is running in 64 bit or
>> compatible mode doesn't change the ABI.
> 
> Agreed. For the hypercall case you're looking at, switching the name to
> is_64bit makes sense.
> 
>> struct compat_shared_info is used only when the guest is running natively in a
>> 32-bit build.
> 
> The struct compat_shared_info is also used in !kvm->arch.xen.long_mode
> on a 64-bit host, as that's what means the guest is considered to be a
> 32-bit guest.
> 
> It's somewhat orthogonal from whether any given vCPU is making any
> given hypercall while in 64-bit mode. The 'long_mode' is *latched* at
> certain specific times which are defined by Xen's historical behaviour.
> 
> I'm suggesting that you clean up longmode→is_64bit for the *hypercalls*
> but leave 'long_mode' as is.
> 

Yes, will only do it for is_64_bit_hypercall().

>