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Mon, 12 Feb 2024 09:05:04 -0800 (PST) Date: Mon, 12 Feb 2024 10:05:02 -0700 From: Alex Williamson To: Cc: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [PATCH v7 4/4] vfio: convey kvm that the vfio-pci device is wc safe Message-ID: <20240212100502.2b5009e4.alex.williamson@redhat.com> In-Reply-To: <20240211174705.31992-5-ankita@nvidia.com> References: <20240211174705.31992-1-ankita@nvidia.com> <20240211174705.31992-5-ankita@nvidia.com> X-Mailer: Claws Mail 4.2.0 (GTK 3.24.38; x86_64-redhat-linux-gnu) 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-Transfer-Encoding: 7bit On Sun, 11 Feb 2024 23:17:05 +0530 wrote: > From: Ankit Agrawal > > The code to map the MMIO in S2 as NormalNC is enabled when conveyed > that the device is WC safe using a new flag VM_ALLOW_ANY_UNCACHED. > > Make vfio-pci set the VM_ALLOW_ANY_UNCACHED flag. > > This could be extended to other devices in the future once that > is deemed safe. > > Suggested-by: Catalin Marinas > Acked-by: Jason Gunthorpe > Acked-by: Catalin Marinas > Signed-off-by: Ankit Agrawal > --- > drivers/vfio/pci/vfio_pci_core.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c > index 1cbc990d42e0..eba2146202f9 100644 > --- a/drivers/vfio/pci/vfio_pci_core.c > +++ b/drivers/vfio/pci/vfio_pci_core.c > @@ -1862,8 +1862,12 @@ int vfio_pci_core_mmap(struct vfio_device *core_vdev, struct vm_area_struct *vma > /* > * See remap_pfn_range(), called from vfio_pci_fault() but we can't > * change vm_flags within the fault handler. Set them now. > + * > + * Set an additional flag VM_ALLOW_ANY_UNCACHED to convey kvm that > + * the device is wc safe. > */ That's a pretty superficial comment. Check that this is accurate, but maybe something like: The VM_ALLOW_ANY_UNCACHED flag is implemented for ARM64, allowing stage 2 device mapping attributes to use Normal-NC rather than DEVICE_nGnRE, which allows guest mappings supporting combining attributes (WC). This attribute has potential risks with the GICv2 VCPU interface, but is expected to be safe for vfio-pci use cases. And specifically, I think these other devices that may be problematic as described in the cover letter is a warning against use for vfio-platform, is that correct? Thanks, Alex > - vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP); > + vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED | VM_IO | VM_PFNMAP | > + VM_DONTEXPAND | VM_DONTDUMP); > vma->vm_ops = &vfio_pci_mmap_ops; > > return 0;