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

[Jon Kohler <jon@nutanix.com>]

> On Oct 20, 2025, at 3:53 PM, Pawan Gupta <pawan.kumar.gupta@linux.intel.com> wrote:
> 
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> On Mon, Oct 20, 2025 at 07:38:40PM +0000, Jon Kohler wrote:
>> 
>> 
>>> On Oct 20, 2025, at 3:26 PM, Pawan Gupta <pawan.kumar.gupta@linux.intel.com> wrote:
>>> 
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>>> CAUTION: External Email
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>>> 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://urldefense.proofpoint.com/v2/url?u=https-3A__lore.kernel.org_all_8c1797e488b42650f62d816f25c58726eb522fad.1745946029.git.pawan.kumar.gupta-40linux.intel.com_&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=NGPRGGo37mQiSXgHKm5rCQ&m=VbwUoxssQYa-v84la6cXovWYpxyF8bkkceV-vYCb49Uub4S7nrlIY9X_76fNKHh4&s=4EIqw5U1jEwif_3zPCXppj9ss0-Ogv1cFVUscBE7hxU&e=
>> 
>> 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.
> 
> So this check isn't working?
> 
> kvm_get_arch_capabilities()
> {
> ...
>         if (!boot_cpu_has_bug(X86_BUG_ITS))
>                 data |= ARCH_CAP_ITS_NO;

Ah! Ok, let me triple check the system under test and get back to you