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

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Message-ID: <20251020195331.nnnp4y4o3fyzgwvx@desk>
Date: Mon, 20 Oct 2025 12:53:31 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Jon Kohler <jon@...anix.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 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@...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.

So this check isn't working?

kvm_get_arch_capabilities()
{
...
         if (!boot_cpu_has_bug(X86_BUG_ITS))
                 data |= ARCH_CAP_ITS_NO;