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From: Thomas Gleixner <tglx@linutronix.de>
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To: K Prateek Nayak <kprateek.nayak@amd.com>, Ingo Molnar
 <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, Dave Hansen
 <dave.hansen@linux.intel.com>, Sean Christopherson <seanjc@google.com>,
 Paolo Bonzini <pbonzini@redhat.com>, x86@kernel.org
Cc: Naveen rao <naveen.rao@amd.com>, Sairaj Kodilkar <sarunkod@amd.com>, "H.
 Peter Anvin" <hpa@zytor.com>, "Peter Zijlstra (Intel)"
 <peterz@infradead.org>, "Xin Li (Intel)" <xin@zytor.com>, Pawan Gupta
 <pawan.kumar.gupta@linux.intel.com>, linux-kernel@vger.kernel.org,
 kvm@vger.kernel.org, Mario Limonciello <mario.limonciello@amd.com>,
 "Gautham R. Shenoy" <gautham.shenoy@amd.com>, Babu Moger
 <babu.moger@amd.com>, Suravee Suthikulpanit
 <suravee.suthikulpanit@amd.com>, K Prateek Nayak <kprateek.nayak@amd.com>
Subject: Re: [PATCH v3 4/4] x86/cpu/topology: Check for
 X86_FEATURE_XTOPOLOGY instead of passing has_topoext
In-Reply-To: <87ms7o3kn6.ffs@tglx>
References: <20250818060435.2452-1-kprateek.nayak@amd.com>
 <20250818060435.2452-5-kprateek.nayak@amd.com> <87ms7o3kn6.ffs@tglx>
Date: Sun, 24 Aug 2025 16:55:24 +0200
Message-ID: <87jz2s3h2b.ffs@tglx>
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On Sun, Aug 24 2025 at 15:38, Thomas Gleixner wrote:
>> -	if (!has_topoext) {
>> +	if (!cpu_feature_enabled(X86_FEATURE_XTOPOLOGY)) {
>>  		/*
>>  		 * Prefer initial_apicid parsed from XTOPOLOGY leaf
>>  		 * 0x8000026 or 0xb if available. Otherwise prefer the
>
> That's patently wrong.
>
> The leaves might be "available", but are not guaranteed to be valid. So
> FEATURE_XTOPOLOGY gives you the wrong answer.
>
> The has_topoext logic is there for a reason.

Hrm. I have to correct myself. It's set by the 0xb... parsing when that
finds a valid leaf. My memory tricked me on that.

So yes, it can be used for that, but that's a cleanup. The simple fix
should be applied first as that's trivial to backport.