From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7049F45975; Wed, 6 Dec 2023 16:31:55 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 942DDC433CC; Wed, 6 Dec 2023 16:31:50 +0000 (UTC) Date: Wed, 6 Dec 2023 16:31:48 +0000 From: Catalin Marinas To: Jason Gunthorpe Cc: Marc Zyngier , ankita@nvidia.com, Shameerali Kolothum Thodi , oliver.upton@linux.dev, suzuki.poulose@arm.com, yuzenghui@huawei.com, will@kernel.org, ardb@kernel.org, akpm@linux-foundation.org, gshan@redhat.com, aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com, vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com, jhubbard@nvidia.com, danw@nvidia.com, mochs@nvidia.com, kvmarm@lists.linux.dev, kvm@vger.kernel.org, lpieralisi@kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory Message-ID: References: <20231205130517.GD2692119@nvidia.com> <20231205164318.GG2692119@nvidia.com> <86bkb4bn2v.wl-maz@kernel.org> <86a5qobkt8.wl-maz@kernel.org> <868r67blwo.wl-maz@kernel.org> <20231206151603.GR2692119@nvidia.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231206151603.GR2692119@nvidia.com> On Wed, Dec 06, 2023 at 11:16:03AM -0400, Jason Gunthorpe wrote: > On Wed, Dec 06, 2023 at 12:14:18PM +0000, Catalin Marinas wrote: > > We could do with a pgprot_maybewritecombine() or > > pgprot_writecombinenospec() (similar to Jason's idea but without > > changing the semantics of pgprot_device()). For the user mapping on > > arm64 this would be Device (even _GRE) since it can't disable > > speculation but stage 2 would leave the decision to the guest since the > > speculative loads aren't much different from committed loads done > > wrongly. > > This would be fine, as would a VMA flag. Please pick one :) > > I think a VMA flag is simpler than messing with pgprot. I guess one could write a patch and see how it goes ;). > > If we want the VMM to drive this entirely, we could add a new mmap() > > flag like MAP_WRITECOMBINE or PROT_WRITECOMBINE. They do feel a bit > > As in the other thread, we cannot unconditionally map NORMAL_NC into > the VMM. I'm not suggesting this but rather the VMM map portions of the BAR with either Device or Normal-NC, concatenate them (MAP_FIXED) and pass this range as a memory slot (or multiple if a slot doesn't allow multiple vmas). > > The latter has some benefits for DPDK but it's a lot more involved > > with > > DPDK WC support will be solved with some VFIO-only change if anyone > ever cares to make it, if that is what you mean. Yeah. Some arguments I've heard in private and public discussions is that the KVM device pass-through shouldn't be different from the DPDK case. So fixing that would cover KVM as well, though we'd need additional logic in the VMM. BenH had a short talk at Plumbers around this - https://youtu.be/QLvN3KXCn0k?t=7010. There was some statement in there that for x86, the guests are allowed to do WC without other KVM restrictions (not sure whether that's the case, not familiar with it). > > having to add device-specific knowledge into the VMM. The VMM would also > > have to present the whole BAR contiguously to the guest even if there > > are different mapping attributes within the range. So a lot of MAP_FIXED > > uses. I'd rather leaving this decision with the guest than the VMM, it > > looks like more hassle to create those mappings. The VMM or the VFIO > > could only state write-combine and speculation allowed. > > We talked about this already, the guest must decide, the VMM doesn't > have the information to pre-predict which pages the guest will want to > use WC on. Are the Device/Normal offsets within a BAR fixed, documented in e.g. the spec or this is something configurable via some MMIO that the guest does. -- Catalin