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Wed, 17 Jun 2020 13:46:54 +0000 (UTC) Date: Wed, 17 Jun 2020 14:46:52 +0100 From: "Dr. David Alan Gilbert" To: Laszlo Ersek Subject: Re: ovmf / PCI passthrough impaired due to very limiting PCI64 aperture Message-ID: <20200617134652.GE2776@work-vm> References: <99779e9c-f05f-501b-b4be-ff719f140a88@canonical.com> <20200616165043.24y2cp53axk7uggy@sirius.home.kraxel.org> <20200616165746.GH2788@work-vm> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.14.0 (2020-05-02) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.120; envelope-from=dgilbert@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/16 23:30:45 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action 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: Pedro Principeza , ehabkost@redhat.com, Dann Frazier , Guilherme Piccoli , qemu-devel@nongnu.org, Christian Ehrhardt , Gerd Hoffmann , fw@gpiccoli.net Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Laszlo Ersek (lersek@redhat.com) wrote: > On 06/16/20 19:14, Guilherme Piccoli wrote: > > Thanks Gerd, Dave and Eduardo for the prompt responses! > > > > So, I understand that when we use "-host-physical-bits", we are > > passing the *real* number for the guest, correct? So, in this case we > > can trust that the guest physbits matches the true host physbits. > > > > What if then we have OVMF relying in the physbits *iff* > > "-host-phys-bits" is used (which is the default in RH and a possible > > machine configuration on libvirt XML in Ubuntu), and we have OVMF > > fallbacks to 36-bit otherwise? > > I've now read the commit message on QEMU commit 258fe08bd341d, and the > complexity is simply stunning. > > Right now, OVMF calculates the guest physical address space size from > various range sizes (such as hotplug memory area end, default or > user-configured PCI64 MMIO aperture), and derives the minimum suitable > guest-phys address width from that address space size. This width is > then exposed to the rest of the firmware with the CPU HOB (hand-off > block), which in turn controls how the GCD (global coherency domain) > memory space map is sized. Etc. > > If QEMU can provide a *reliable* GPA width, in some info channel (CPUID > or even fw_cfg), then the above calculation could be reversed in OVMF. > We could take the width as a given (-> produce the CPU HOB directly), > plus calculate the *remaining* address space between the GPA space size > given by the width, and the end of the memory hotplug area end. If the > "remaining size" were negative, then obviously QEMU would have been > misconfigured, so we'd halt the boot. Otherwise, the remaining area > could be used as PCI64 MMIO aperture (PEI memory footprint of DXE page > tables be darned). > > > Now, regarding the problem "to trust or not" in the guests' physbits, > > I think it's an orthogonal discussion to some extent. It'd be nice to > > have that check, and as Eduardo said, prevent migration in such cases. > > But it's not really preventing OVMF big PCI64 aperture if we only > > increase the aperture _when "-host-physical-bits" is used_. > > I don't know what exactly those flags do, but I doubt they are clearly > visible to OVMF in any particular way. The firmware should trust whatever it reads from the cpuid and thus gets told from qemu; if qemu is doing the wrong thing there then that's our problem and we need to fix it in qemu. Dave > Thanks > Laszlo -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK