From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D70A8C4332F for ; Sun, 13 Nov 2022 13:33:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235271AbiKMNdA (ORCPT ); Sun, 13 Nov 2022 08:33:00 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47230 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231972AbiKMNc7 (ORCPT ); Sun, 13 Nov 2022 08:32:59 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C687AFAC5; Sun, 13 Nov 2022 05:32:56 -0800 (PST) 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=in2O/a9qLvwFLNtcB3MFl0zAbBPnQnAnMBCpvQ7sLZs=; b=pNYH9TaHa1HPTpXcZCMpqJiJat /4QkGgDd8lQitCHSTroMYs18UYlXUoOTaapULf/HabConDMBDKo0a0LI9pTSP55R/+3x42+fK+VGF dA3w+icwpx/i7jpysZdhAYLAk4i5nWYnxQDiACYUg3Qii5t0ex/HjbG/G7HBJpdOwGqQjCci1KuSV XhtwUcYiyM7tcgtBUQ4LE2gW2xjwhCxifiwVTXlU9Kprnppncjhec65f5pAJzjXCR2E7wxVFI9oHR PheKFV+Jl/rFBVZRiXeffuoHB3pwCWvPfrqt+7/X+ZwqufazbWR1FiXR4Q4ECw8egE4x1SOBoboDc CalI5rPg==; Received: from 54-240-197-228.amazon.com ([54.240.197.228] helo=freeip.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1ouD6U-00EgFg-N9; Sun, 13 Nov 2022 13:32:55 +0000 Message-ID: Subject: Re: [PATCH 03/16] KVM: x86: set gfn-to-pfn cache length consistently with VM word size From: David Woodhouse To: Paolo Bonzini , Sean Christopherson Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, mhal@rbox.co, Paul Durrant , Metin Kaya Date: Sun, 13 Nov 2022 13:32:46 +0000 In-Reply-To: References: <20221027161849.2989332-1-pbonzini@redhat.com> <20221027161849.2989332-4-pbonzini@redhat.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-DiOVFhyRoAb889tkGkTb" User-Agent: Evolution 3.36.5-0ubuntu1 MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-DiOVFhyRoAb889tkGkTb Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2022-10-28 at 13:36 +0200, Paolo Bonzini wrote: > We _should_ be following the Xen API, which does not even say that the > areas have to fit in a single page. In fact, even Linux's >=20 > struct vcpu_register_runstate_memory_area area; >=20 > area.addr.v =3D &per_cpu(xen_runstate, cpu); > if (HYPERVISOR_vcpu_op(VCPUOP_register_runstate_memory_area, > xen_vcpu_nr(cpu), &area)) >=20 > could fail or not just depending on the linker's whims, if I'm not > very confused. >=20 > Other data structures *do* have to fit in a page, but the runstate area > does not and it's exactly the one where the cache comes the most handy. > For this I'm going to wait for David to answer. Yeah, I recall vetting a bunch of these to ensure that it's safe to assume that it does fit within the page... but that clearly isn't true for the runstate_area. As things stand, I believe a guest is perfectly entitled to provide a region which crosses a page boundary, and Xen copes with that. But as you say, KVM doesn't. However, I don't think this *is* the case where the cache comes in the most handy. The cache is really useful where we have to do *atomic* operations on guest addresses, and doing so directly is a whole lot nicer than futex-style try-it-and-fail-gracefully operations on userspace addresses. For the runstate area, I think we can live with using a gfn_to_hva cache instead, and writing via the userspace address (with appropriate atomicity for the RUNSTATE_runnable case as we have at the moment gating the refresh). --=-DiOVFhyRoAb889tkGkTb Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCEkQw ggYQMIID+KADAgECAhBNlCwQ1DvglAnFgS06KwZPMA0GCSqGSIb3DQEBDAUAMIGIMQswCQYDVQQG EwJVUzETMBEGA1UECBMKTmV3IEplcnNleTEUMBIGA1UEBxMLSmVyc2V5IENpdHkxHjAcBgNVBAoT FVRoZSBVU0VSVFJVU1QgTmV0d29yazEuMCwGA1UEAxMlVVNFUlRydXN0IFJTQSBDZXJ0aWZpY2F0 aW9uIEF1dGhvcml0eTAeFw0xODExMDIwMDAwMDBaFw0zMDEyMzEyMzU5NTlaMIGWMQswCQYDVQQG EwJHQjEbMBkGA1UECBMSR3JlYXRlciBNYW5jaGVzdGVyMRAwDgYDVQQHEwdTYWxmb3JkMRgwFgYD VQQKEw9TZWN0aWdvIExpbWl0ZWQxPjA8BgNVBAMTNVNlY3RpZ28gUlNBIENsaWVudCBBdXRoZW50 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