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Thu, 9 Jan 2020 11:01:28 +0000 (UTC) Subject: Re: [PATCH for-5.0 v11 08/20] virtio-iommu: Implement translate To: Jean-Philippe Brucker References: <20191219144936.GB50561@xz-x1> <9ec9d0d5-062b-f96b-c72c-4d15865ff9a1@redhat.com> <20191220162642.GA2626852@myrica> <20191220165100.GA3780@xz-x1> <20200106170634.GF2062@myrica> <20200106175850.GC219677@xz-x1> <20200107101024.GB832497@myrica> <20200109084700.GA934975@myrica> <0beb9d61-05b8-3152-e967-64413def6ab7@redhat.com> <20200109104032.GA937123@myrica> From: Auger Eric Message-ID: Date: Thu, 9 Jan 2020 12:01:26 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20200109104032.GA937123@myrica> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-MC-Unique: l2CcCSRyO8K9kTXT8J2CWw-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.120 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: yang.zhong@intel.com, peter.maydell@linaro.org, kevin.tian@intel.com, tnowicki@marvell.com, mst@redhat.com, jean-philippe.brucker@arm.com, quintela@redhat.com, qemu-devel@nongnu.org, Peter Xu , armbru@redhat.com, bharatb.linux@gmail.com, qemu-arm@nongnu.org, dgilbert@redhat.com, eric.auger.pro@gmail.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Hi, On 1/9/20 11:40 AM, Jean-Philippe Brucker wrote: > On Thu, Jan 09, 2020 at 09:58:49AM +0100, Auger Eric wrote: >>>> I share Peter's concern about having a different default policy than x86. >>> >>> Yes I'd say just align with whatever policy is already in place. Do you >>> think we could add a command-line option to let people disable >>> default-bypass, though? That would be a convenient way to make the IOMMU >>> protection airtight for those who need it. >> Yes I could easily add a device option to disable the default bypass. >> >> Shall we change the meaning of the F_BYPASS feature then? If exposed by >> the device, the device does bypass by default, otherwise it doesn't. >> This would be controlled by the device option. > > For a device that doesn't do bypass by default, the driver wouldn't have > the ability to enable bypass (feature bit not offered, not negotiable). > >> The driver then could have means to overwrite this behavior once loaded? > > Let's keep the bypass feature bit for this. If the bit is offered, the > driver chooses to enable or disable it. If the bit is not offered or not > negotiated, then the behavior is deny. If the bit is offered and > negotiated then the behavior is allow. > > We can say that before features negotiation (latched at features register > write, I think, in practice?) the behavior is platform dependent. The > current wording about bypass intends to discourage unsafe choices but > makes a strong statement only about the device behavior after features > negotiation. OK. May be worth adding in the spec later. By the way what is the plan for the vote? Thanks Eric > > We could add a second feature bit specifically for the boot bypass > behavior. It wouldn't be useful to the OS (the driver doesn't have a > choice) but could present a bit in config space that allows a firmware to > disable boot-bypass in a way that is sticky across reset. So when the OS > resets the device after taking it over, it doesn't accidentally enable > bypass. I wouldn't bother though. If a FW/bootloader is able to support > virtio-iommu, the user might as well instantiate the device with the > default-deny option. > > Thanks, > Jean > >