linux-kernel - Re: [PATCH] x86/its: use Sapphire Rapids+ feature to opt out

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Message-ID: <7985318A-3669-4A1C-9282-D940F142252F@nutanix.com>
Date: Mon, 20 Oct 2025 19:38:40 +0000
From: Jon Kohler <jon@...anix.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
CC: Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson
	<seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov
	<bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf
	<jpoimboe@...nel.org>,
        Jonathan Corbet <corbet@....net>, Ingo Molnar
	<mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Brian
 Gerst <brgerst@...il.com>,
        Brendan Jackman <jackmanb@...gle.com>,
        "Ahmed S.
 Darwish" <darwi@...utronix.de>,
        Alexandre Chartre
	<alexandre.chartre@...cle.com>,
        "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/its: use Sapphire Rapids+ feature to opt out



> On Oct 20, 2025, at 3:26 PM, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com> wrote:
> 
> !-------------------------------------------------------------------|
>  CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> On Mon, Oct 20, 2025 at 09:21:41AM -0700, Dave Hansen wrote:
>> On 10/20/25 09:05, Jon Kohler wrote:
>>> Was running into some testing issues with my qemu change internally,
>>> but I’ll get that out this week once I clear them.
>> 
>> BTW, if there are folks out there that are working on things like QEMU
>> and want more formal or regular notification from vendors that a FOO_NO
>> bit has been added, that can probably be arranged.
>> 
>> The real issue here is that nobody cared enough to get QEMU to
>> comprehend ITS_NO right after the embargo was lifted, right?
> 
> ITS_NO support was added to QEMU right after the embargo was lifted:
> 
> https://lore.kernel.org/all/8c1797e488b42650f62d816f25c58726eb522fad.1745946029.git.pawan.kumar.gupta@linux.intel.com/

Pawan - I saw that, but I wasn’t able to get that to work, as the supported
feature checker will fail, and the VM will fail to start.

Specifically, kvm_arch_get_supported_msr_feature() will not show it as a
“supported” bit, and kick it back, and you’ll get an error like so when setting
-cpu … “,its-no=yes"

qemu-kvm: warning: host doesn't support requested feature: MSR(10AH).its-no [bit 62]

That’s because qemu queries KVM side, which is checking against supported
hardware bits. Since this doesn’t come from hardware, it goes boom.

I ran into something similar when dealing with the fb-clear emulation [1] that I added to
qemu a week or so ago, and was thinking of something something similar-ish to
get around just this same issue with its-no. I need to finish cooking that up and
I’ll CC you on the QEMU side fix.

[1] https://github.com/qemu/qemu/commit/00001a22d183ce96c110690987bf9dd6a8548552