linux-kernel - Re: [PATCH 00/19] Refresh queued CET virtualization series

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Date:   Sat, 18 Jun 2022 14:43:19 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     seanjc@...gle.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, rick.p.edgecombe@...el.com
Subject: Re: [PATCH 00/19] Refresh queued CET virtualization series


On 6/16/2022 11:28 PM, Paolo Bonzini wrote:
> On 6/16/22 16:18, Peter Zijlstra wrote:
>> On Thu, Jun 16, 2022 at 12:21:20PM +0200, Paolo Bonzini wrote:
>>> On 6/16/22 12:12, Peter Zijlstra wrote:
>>>> Do I understand this right in that a host without X86_KERNEL_IBT 
>>>> cannot
>>>> run a guest with X86_KERNEL_IBT on? That seems unfortunate, since that
>>>> was exactly what I did while developing the X86_KERNEL_IBT patches.
>>>>
>>>> I'm thinking that if the hardware supports it, KVM should expose it,
>>>> irrespective of the host kernel using it.
>>>
>>> For IBT in particular, I think all processor state is only loaded 
>>> and stored
>>> at vmentry/vmexit (does not need XSAVES), so it should be feasible.
>>
>> That would be the S_CET stuff, yeah, that's VMCS managed. The U_CET
>> stuff is all XSAVE though.
>
> What matters is whether XFEATURE_MASK_USER_SUPPORTED includes 
> XFEATURE_CET_USER. 

Small correction, XFEATURE_CET_USER belongs to 
XFEATURE_MASK_SUPERVISOR_SUPPORTED, the name is misleading.


> If you build with !X86_KERNEL_IBT, KVM can still rely on the FPU state 
> for U_CET state, and S_CET is saved/restored via the VMCS independent 
> of X86_KERNEL_IBT.

A fundamental question is, should KVM always honor host CET enablement 
before expose the feature to guest? i.e., check X86_KERNEL_IBT and 
X86_SHADOW_STACK.


>
> Paolo
>
>> But funny thing, CPUID doesn't enumerate {U,S}_CET separately. It *does*
>> enumerate IBT and SS separately, but for each IBT/SS you have to
>> implement both U and S.
>>
>> That was a problem with the first series, which only implemented support
>> for U_CET while advertising IBT and SS (very much including S_CET), and
>> still is a problem with this series because S_SS is missing while
>> advertised.
>