From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9363820311; Tue, 2 Sep 2025 08:31:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756801906; cv=none; b=ESHPjP231u/u+tfjIRwSShQW/cbwCmQkQrLOIfu7CmrO6kzvXT9RBxKaK5lUR5D5JgUHSahnYzdInL3nEYTGKheVJ601NDa2zMPSci7BBNxuud3nfs8mnmh2om3Xe6vJlbk52wHh1FOPBgQhX9Pc2pqq3gKx2lI2zHuuKcuhqdk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756801906; c=relaxed/simple; bh=7s3kocN5f9Dp03aVdzKM1kW8L/5pmll74PuYJbceKQo=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=O+Cqdi3FruDQhoxYiH+BRI/RV9VDY1RQR/hP7WR1zzSeHqxVHfi5zsyHi40uZBXRd2W9oE0O1/MwlIaujncrkWFWYhNq2u8Fe4G8SX9HS2pH00Ub1jd986ldafZ1pO6Wbq1hhxxi4GDt6mnsXhNywCz475mGUukoG5BKXfZA2Kg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=casper.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=ddkHb3nJ; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=casper.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="ddkHb3nJ" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=7s3kocN5f9Dp03aVdzKM1kW8L/5pmll74PuYJbceKQo=; b=ddkHb3nJL11eyxln//jjhNS04M JFzYuS1I3q7BPHseJRczzdSgFD/iS9gJO8SZf7b+qt8FjHsLwzkWyjHVD3TqoANSc5sxFgikqgZYh YXKhBJnhWnqa1xaBoG8knNHT70Qb4UnoKDiLpBwwI5HZ5IeaGqmAgLFm+TEb4gvTZAyIo2+YHiYbD XmFhYl6N+/xfAXOge36JL5+x5yxcySfRzmPpysPDaHDa8GUr2WE0GncNgh9Yiab1K0ni3+SimJ582 13sDSXDxX1Na+rb+5mL7NIkSuUGsZigvQ8HNfiEdwKzJgFkVZrhcwO/lwy9fcOiSlWubR8R7Ubbm4 GarzXkyg==; Received: from 54-240-197-239.amazon.com ([54.240.197.239] helo=freeip.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1utMQN-00000003CHq-3Rz0; Tue, 02 Sep 2025 08:31:32 +0000 Message-ID: Subject: Re: [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest and host From: David Woodhouse To: Sean Christopherson Cc: Paul Durrant , Fred Griffoul , Colin Percival , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "x86@kernel.org" , "H. Peter Anvin" , Vitaly Kuznetsov , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "Graf (AWS), Alexander" , Ajay Kaher , Alexey Makhalov Date: Tue, 02 Sep 2025 09:31:31 +0100 In-Reply-To: References: <6783241f1bfadad8429f66c82a2f8810a74285a0.camel@infradead.org> <01000198cf7ec03e-dfc78632-42ee-480b-8b51-3446fbb555d1-000000@email.amazonses.com> <54BCC060-1C9B-4BE4-8057-0161E816A9A3@amazon.co.uk> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-Z6kcJKZlVoILQqjcNG0Q" User-Agent: Evolution 3.52.3-0ubuntu1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html --=-Z6kcJKZlVoILQqjcNG0Q Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2025-08-29 at 13:36 -0700, Sean Christopherson wrote: > =C2=A0 > > This does mean userspace would have to set the vCPU's TSC frequency and > > then query the kernel before setting up its CPUID. And in the absence > > of scaling, this KVM API would report the hardware TSC frequency. >=20 > Reporting the hardware TSC frequency on CPUs without scaling seems all ki= nds of > wrong (which another reason I don't like KVM shoving in the state).=C2=A0= Of course, > reporting the frequency KVM is trying to provide isn't great either, as t= he guest > will definitely observe something in between those two. Yes, on CPUs that don't support TSC scaling, we should not attempt to advertise a frequency. Where I said 'in the absence of scaling' I meant modern CPUs but where the VMM just didn't ask for TSC scaling. > > I guess the API would have to return -EHARDWARETOOSTUPID if the TSC fre= quency > > *isn't* the same across all CPUs and all power states, etc. >=20 > What if KVM advertises the flag in KVM_GET_SUPPORTED_CPUID if and only if= the > TSC will be constant from the guest's perspective?=C2=A0 TSC scaling has = been supported > by AMD and Intel for ~10 years, it doesn't seem at all unreasonable to re= strict > the feature to somewhat modern hardware.=C2=A0 And if userspace or the ad= min knows > better than KVM, then userspace can always ignore KVM and report the freq= uency > anyways. I hadn't put it in KVM_GET_SUPPORTED_CPUID; I was following the lead of the existing Xen leaf support, where *if* userspace provides that leaf, KVM will dynamically correct the values in it. The problem is that KVM_GET_SUPPORTED_CPUID is a *system* ioctl on the bare /dev/kvm device, isn 't it? So even if a VMM has set the TSC frequency VM-wide with KVM_SET_TSC_KHZ instead of doing it the old per- vCPU way, how can it get the results for a specific VM? --=-Z6kcJKZlVoILQqjcNG0Q Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 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