linux-kernel - Re: [PATCH v9 17/22] KVM: x86: Advertise support for FRED

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Message-ID: <04d96812-f74a-4f43-9ea4-c4f2723251c5@linux.intel.com>
Date: Tue, 20 Jan 2026 17:46:25 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Xin Li <xin@...or.com>, Chao Gao <chao.gao@...el.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 linux-doc@...r.kernel.org, pbonzini@...hat.com, seanjc@...gle.com,
 corbet@....net, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, luto@...nel.org,
 peterz@...radead.org, andrew.cooper3@...rix.com, hch@...radead.org,
 sohil.mehta@...el.com
Subject: Re: [PATCH v9 17/22] KVM: x86: Advertise support for FRED



On 1/20/2026 5:09 PM, Xin Li wrote:
> 
> 
>> On Jan 20, 2026, at 12:07 AM, Chao Gao <chao.gao@...el.com> wrote:
>>
>> On Mon, Jan 19, 2026 at 10:56:29PM -0800, Xin Li wrote:
>>>
>>>
>>>> On Nov 11, 2025, at 11:30 PM, Chao Gao <chao.gao@...el.com> wrote:
>>>>
>>>> I'm not sure if AMD CPUs support FRED, but just in case, we can clear FRED
>>>> i.e., kvm_cpu_cap_clear(X86_FEATURE_FRED) in svm_set_cpu_caps().
>>>
>>> AMD will support FRED, with ISA level compatibility:
>>>
>>> https://www.amd.com/en/blogs/2025/amd-and-intel-celebrate-first-anniversary-of-x86-ecosys.html
>>>
>>> Thus we don’t need to clear the bit.
>>
>> In this case, we need to clear FRED for AMD.
>>
>> The concern is that before AMD's FRED KVM support is implemented, FRED will be
>> exposed to userspace on AMD FRED-capable hardware. This may cause issues.
> 
> Hmm, I think it’s Qemu does that.
> 
> We have 2 filters, one in Qemu and one in KVM, only both are set a feature is enabled.
> 
> What I have missed?

If a newer QEMU (with AMD's FRED support patch) + an older KVM (without AMD's
FRED support, but KVM advertises it), it may cause issues.

I guess it's no safety issue for host though, AMD should also require some
control bit(s) to be set to allow guests to use the feature.

I agree with Chao that it should be cleared for AMD before AMD's FRED KVM
support is implemented.