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 X-Spam-Level: X-Spam-Status: No, score=-13.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id ECE31C433E6 for ; Fri, 15 Jan 2021 16:46:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BF4B4221FE for ; Fri, 15 Jan 2021 16:46:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726046AbhAOQqb (ORCPT ); Fri, 15 Jan 2021 11:46:31 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37406 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725910AbhAOQqb (ORCPT ); Fri, 15 Jan 2021 11:46:31 -0500 Received: from mail-pg1-x531.google.com (mail-pg1-x531.google.com [IPv6:2607:f8b0:4864:20::531]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1901DC061757 for ; Fri, 15 Jan 2021 08:45:51 -0800 (PST) Received: by mail-pg1-x531.google.com with SMTP id 30so6347742pgr.6 for ; Fri, 15 Jan 2021 08:45:51 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=TWuR3ZJ2UHICr0CiKVPTEVeDZF6pZ+WV9FNSFwhbdvQ=; b=tWvplHkApxVZ6MP6t8FotDQ+IHmJJBEZQWdV4QVV3+cKi3iDm/rTftLgnt/Ui8bEQR gym7ZJ/zjKICKBaXLKWxMtZSkZH+Hse7G9sYW2CRcQ3aDRPmRST+Jn4wfjpQ8bE6SOGV /RIW5lUJC7S6LvIGaFxhubFGwtI3JUMtIqu06WAKBvXiC7M51OWFgxzayu4bNUl6gCct zuaAImQPxpmkgSA3ckGCyCQwDozK8aemjLo2RVeyiXJG5yyOulAGuFs4Uap53ujlm20D dL8ACsVf6hZKZTo0AuoHCyf1bKbeiruCuqHY+ULuzpBFT2/gqocyIU15cXxeEQpH98oI 4hQQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=TWuR3ZJ2UHICr0CiKVPTEVeDZF6pZ+WV9FNSFwhbdvQ=; b=klJQ0k179oQSC7NgI5yfup/cENMIUgbS889DFU0ETwSttQ/xmXreSsEEQUtMv0NR5z jlc7TNx1DHdqViO/brtYlSEo5VB8rHzMACP7nnl9zzpbhw8WS8lXPnbxtuFPlvyXImU8 kmLDFHeDT6Poz65MkHrW1YorbPoTgK41ud9eWiAGtQLmaLsEIq3FKuwABG8lHFKftLWJ zjMljvx0rILgPh1YilDaOP8N3ktusglQX6gevqGqqsHrU/BH2gn64QhDu5FfV1LYygZx jC1t4N/aTVNsByJiCG4szZlH5dGXhPT2ARW64LlTbxReIYrrJNWWwERNbevcl/xP9JEj gKyA== X-Gm-Message-State: AOAM5339rtXVPNSqleIXr9GGvDa6zO57D9u7ASa3OWD0HbzRwXXM3y1G JWwbXio22zpvObdskJfNiySppTbNUYCybw== X-Google-Smtp-Source: ABdhPJx/joI/0iL7ov0niLl5D6IK5weqMVWW7wXnPSsxawzu9EjDeF4O5L36F6bX+KrMnIs12bNXpQ== X-Received: by 2002:a05:6a00:884:b029:1b4:440f:bce7 with SMTP id q4-20020a056a000884b02901b4440fbce7mr2209052pfj.20.1610729150481; Fri, 15 Jan 2021 08:45:50 -0800 (PST) Received: from google.com ([2620:15c:f:10:1ea0:b8ff:fe73:50f5]) by smtp.gmail.com with ESMTPSA id 4sm9199887pjn.14.2021.01.15.08.45.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 15 Jan 2021 08:45:49 -0800 (PST) Date: Fri, 15 Jan 2021 08:45:43 -0800 From: Sean Christopherson To: Vitaly Kuznetsov Cc: kvm@vger.kernel.org, Paolo Bonzini , Wanpeng Li , Jim Mattson Subject: Re: [PATCH RFC 0/4] KVM: x86: Drastically raise KVM_USER_MEM_SLOTS limit Message-ID: References: <20210115131844.468982-1-vkuznets@redhat.com> <87zh1a5fuj.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87zh1a5fuj.fsf@vitty.brq.redhat.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Jan 15, 2021, Vitaly Kuznetsov wrote: > Sean Christopherson writes: > > > On Fri, Jan 15, 2021, Vitaly Kuznetsov wrote: > >> Longer version: > >> > >> Current KVM_USER_MEM_SLOTS limit (509) can be a limiting factor for some > >> configurations. In particular, when QEMU tries to start a Windows guest > >> with Hyper-V SynIC enabled and e.g. 256 vCPUs the limit is hit as SynIC > >> requires two pages per vCPU and the guest is free to pick any GFN for > >> each of them, this fragments memslots as QEMU wants to have a separate > >> memslot for each of these pages (which are supposed to act as 'overlay' > >> pages). > > > > What exactly does QEMU do on the backend? I poked around the code a bit, but > > didn't see anything relevant. > > > > In QEMU's terms it registers memory sub-regions for these two pages (see > synic_update() in hw/hyperv/hyperv.c). Memory for these page-sized > sub-regions is allocated separately so in KVM terms they become > page-sized slots and previously continuous 'system memory' slot breaks > into several slots. Doh, I had a super stale version checked out (2.9.50), no wonder I couldn't find anything. Isn't the memslot approach inherently flawed in that the SynIC is per-vCPU, but memslots are per-VM? E.g. if vCPU1 accesses vCPU0's SynIC GPA, I would expect that to access real memory, not the overlay. Or is there more QEMU magic going on that I'm missing?