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From: Oliver Upton To: Ankit Agrawal Cc: Sean Christopherson , Jason Gunthorpe , Marc Zyngier , Catalin Marinas , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "will@kernel.org" , "ryan.roberts@arm.com" , "shahuang@redhat.com" , "lpieralisi@kernel.org" , "david@redhat.com" , Aniket Agashe , Neo Jia , Kirti Wankhede , "Tarun Gupta (SW-GPU)" , Vikram Sethi , Andy Currid , Alistair Popple , John Hubbard , Dan Williams , Zhi Wang , Matt Ochs , Uday Dhoke , Dheeraj Nigam , Krishnakant Jaju , "alex.williamson@redhat.com" , "sebastianene@google.com" , "coltonlewis@google.com" , "kevin.tian@intel.com" , "yi.l.liu@intel.com" , "ardb@kernel.org" , "akpm@linux-foundation.org" , "gshan@redhat.com" , "linux-mm@kvack.org" , "ddutile@redhat.com" , "tabba@google.com" , "qperret@google.com" , "kvmarm@lists.linux.dev" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags Message-ID: References: <86y0wrlrxt.wl-maz@kernel.org> <86wmcbllg2.wl-maz@kernel.org> <20250331145643.GF10839@nvidia.com> <20250407161540.GG1557073@nvidia.com> Precedence: bulk X-Mailing-List: linux-kernel@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: X-Migadu-Flow: FLOW_OUT Hi, On Wed, Apr 16, 2025 at 08:51:05AM +0000, Ankit Agrawal wrote: > Hi, summarizing the discussion so far and outlining the next steps. The key points > are as follows: > 1. KVM cap to expose whether the kernel supports mapping cacheable PFNMAP: > If the host doesn't have FWB, then the capability doesn't exist. Jason, Oliver, Caitlin > and Sean points that this may not be required as userspace do not have > much choice anyways. KVM has to follow the PTEs and userspace cannot ask > for something different. However, Marc points that enumerating FWB support > would allow userspace to discover the support and prevent live-migration > across FWB and non-FWB hosts. Jason suggested that this may still be fine as > we have already built in VFIO side protection where a live migration can be > attempted and then fail because of late-detected HW incompatibilities. > > 2. New memslot flag that VMM passes at memslot registration: > Discussion point that this is not necessary and KVM should just follow the > VMA pgprot. > > 3. Fallback path handling for PFNMAP when the FWB is not set: > Discussion points that there shouldn't be any fallback path and the memslot > should just fail. i.e. KVM should not allow degrading cachable to non-cachable > when it can't do flushing. This is to prevent the potential security issue > pointed by Jason (S1 cacheable, S2 noncacheable). > > > So AIU, the next step is to send out the updated series with the following patches: > 1. Block cacheable PFN map in memslot creation (kvm_arch_prepare_memory_region) > and during fault handling (user_mem_abort()). Yes, we need to prevent the creation of stage-2 mappings to PFNMAP memory that uses cacheable attributes in the host stage-1. I believe we have alignment that this is a bugfix. > 2. Enable support for cacheable PFN maps if S2FWB is enabled by following > the vma pgprot (this patch). > > 3. Add and expose the new KVM cap to expose cacheable PFNMAP (set to false > for !FWB), pending maintainers' feedback on the necessity of this capability. Regarding UAPI: I'm still convinced that we need the VMM to buy in to this behavior. And no, it doesn't matter if this is some VFIO-based mapping or kernel-managed memory. The reality is that userspace is an equal participant in remaining coherent with the guest. Whether or not FWB is employed for a particular region of IPA space is useful information for userspace deciding what it needs to do to access guest memory. Ignoring the Nvidia widget for a second, userspace also needs to know this for 'normal', kernel-managed memory so it understands what CMOs may be necessary when (for example) doing live migration of the VM. So this KVM CAP needs to be paired with a memslot flag. - The capability says KVM is able to enforce Write-Back at stage-2 - The memslot flag says userspace expects a particular GFN range to guarantee Write-Back semantics. This can be applied to 'normal', kernel-managed memory and PFNMAP thingies that have cacheable attributes at host stage-1. - Under no situation do we allow userspace to create non-cacheable mapping at stage-2 for something PFNMAP cacheable at stage-1. No matter what, my understanding is that we all agree the driver which provided the host stage-1 mapping is the authoritative source for memory attributes compatible with a given device. The accompanying UAPI is necessary for the VMM to understand how to handle arbitrary cacheable mappings provided to the VM. Thanks, Oliver