linux-kernel - Re: [PATCH v5 17/19] KVM:x86: Enable guest CET supervisor xstate bit support

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Date:   Wed, 9 Aug 2023 14:07:29 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     <rick.p.edgecombe@...el.com>, <chao.gao@...el.com>,
        <binbin.wu@...ux.intel.com>, <seanjc@...gle.com>,
        <peterz@...radead.org>, <john.allen@....com>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 17/19] KVM:x86: Enable guest CET supervisor xstate bit
 support

On 8/5/2023 6:02 AM, Paolo Bonzini wrote:
> On 8/3/23 06:27, Yang Weijiang wrote:
>>       if (boot_cpu_has(X86_FEATURE_XSAVES)) {
>> +        u32 eax, ebx, ecx, edx;
>> +
>> +        cpuid_count(0xd, 1, &eax, &ebx, &ecx, &edx);
>>           rdmsrl(MSR_IA32_XSS, host_xss);
>>           kvm_caps.supported_xss = host_xss & KVM_SUPPORTED_XSS;
>> +        if (ecx & XFEATURE_MASK_CET_KERNEL)
>> +            kvm_caps.supported_xss |= XFEATURE_MASK_CET_KERNEL;
>>       }
>
> This is a bit hackish and makes me lean more towards adding support for XFEATURE_MASK_CET_KERNEL in host MSR_IA32_XSS (and then possibly hide it in the actual calls to XSAVE/XRSTORS for non-guest FPU).
Yes, if kernel can support CET_U/S bits in XSS, things would be much easier.
But if CET_S bit cannot be enabled for somehow,  we may have KVM emulation
for it.
> Paolo
>