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

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Message-Id: <C3F658E2-BB0D-4461-8412-F4BC5BCB2298@zytor.com>
Date: Tue, 20 Jan 2026 01:09:27 -0800
From: Xin Li <xin@...or.com>
To: 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 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?