linux-kernel - Re: [RFC PATCH v5 089/104] KVM: TDX: Add a placeholder for handler of TDX hypercalls (TDG.VP.VMCALL)

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Date:   Thu, 7 Apr 2022 20:04:18 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Kai Huang <kai.huang@...el.com>, isaku.yamahata@...el.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        isaku.yamahata@...il.com, Jim Mattson <jmattson@...gle.com>,
        erdemaktas@...gle.com, Connor Kuehl <ckuehl@...hat.com>
Subject: Re: [RFC PATCH v5 089/104] KVM: TDX: Add a placeholder for handler of
 TDX hypercalls (TDG.VP.VMCALL)

On 4/7/22 16:39, Sean Christopherson wrote:
> On Thu, Apr 07, 2022, Paolo Bonzini wrote:
>> On 4/7/22 06:15, Kai Huang wrote:
>>>> +static int handle_tdvmcall(struct kvm_vcpu *vcpu)
>>>> +{
>>>> +	struct vcpu_tdx *tdx = to_tdx(vcpu);
>>>> +
>>>> +	if (unlikely(tdx->tdvmcall.xmm_mask))
>>>> +		goto unsupported;
>>> Put a comment explaining this logic?
>>>
>>
>> This only seems to be necessary for Hyper-V hypercalls, which however are
>> not supported by this series in TDX guests (because the kvm_hv_hypercall
>> still calls kvm_*_read, likewise for Xen).
>>
>> So for now this conditional can be dropped.
> 
> I'd prefer to keep the sanity check, it's a cheap and easy way to detect a clear
> cut guest bug.

I don't think it's necessarily a guest bug, just silly but valid behavior.

Paolo

   E.g. KVM would be within its rights to write garbage the XMM
> registers in this case.  Even though KVM isn't to be trusted, KVM can still be
> nice to the guest.