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[79.181.91.42]) by smtp.gmail.com with ESMTPSA id t197sm21921007qke.2.2019.07.25.10.33.52 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Thu, 25 Jul 2019 10:33:55 -0700 (PDT) Date: Thu, 25 Jul 2019 13:33:49 -0400 From: "Michael S. Tsirkin" To: Paolo Bonzini Message-ID: <20190725132521-mutt-send-email-mst@kernel.org> References: <20190719102915.GG18585@stefanha-x1.localdomain> <8736j2p22w.fsf@redhat.com> <904248411098104fcf7db22382172057e50db76c.camel@intel.com> <87tvbdrvin.fsf@redhat.com> <20190725104331-mutt-send-email-mst@kernel.org> <6d318abf-4afa-a1dc-a4e8-3a2d0a6de297@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6d318abf-4afa-a1dc-a4e8-3a2d0a6de297@redhat.com> X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.85.222.193 Subject: Re: [Qemu-devel] [PATCH v3 0/4] Introduce the microvm machine type X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "ehabkost@redhat.com" , Sergio Lopez , "maran.wilson@oracle.com" , "Montes, Julio" , Stefan Hajnoczi , "qemu-devel@nongnu.org" , "kraxel@redhat.com" , "rth@twiddle.net" , "sgarzare@redhat.com" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Jul 25, 2019 at 05:35:01PM +0200, Paolo Bonzini wrote: > On 25/07/19 16:46, Michael S. Tsirkin wrote: > > Actually, I think I have a better idea. > > At the moment we just get an exit on these reads and return all-ones. > > Yes, in theory there could be a UR bit set in a bunch of > > registers but in practice no one cares about these, > > and I don't think we implement them. > > So how about mapping a single page, read-only, and filling it > > with all-ones? > > Yes, that's nice indeed. :) But it does have some cost, in terms of > either number of VMAs or QEMU RSS since the MMCONFIG area is large. > > What breaks if we return all zeroes? Zero is not a valid vendor ID. > > Paolo It isn't but that's not what baremetal does. So there's some risk there ... Why is all zeroes better? We still need to map it, right? -- MST