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[2003:cb:c73d:2400:3be1:a856:724c:fd29]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a099ae7a46sm9423271f8f.44.2025.05.05.01.01.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 05 May 2025 01:01:29 -0700 (PDT) Message-ID: <7e32aabe-c170-4cfc-99aa-f257d2a69364@redhat.com> Date: Mon, 5 May 2025 10:01:26 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 06/13] KVM: x86: Generalize private fault lookups to guest_memfd fault lookups To: Ackerley Tng , Sean Christopherson Cc: Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com, peterx@redhat.com, pankaj.gupta@amd.com References: <386c1169-8292-43d1-846b-c50cbdc1bc65@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: E0BE41A000F X-Stat-Signature: kagex7xejsxzwbrgoc99r6k7bqoy4asm X-Rspam-User: X-HE-Tag: 1746432093-893815 X-HE-Meta: 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 8O1RlP7J 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 03.05.25 00:00, Ackerley Tng wrote: > Sean Christopherson writes: > >> On Fri, May 02, 2025, David Hildenbrand wrote: >>> On 30.04.25 20:58, Ackerley Tng wrote: >>>>> - if (is_private) >>>>> + if (is_gmem) >>>>> return max_level; >>>> >>>> I think this renaming isn't quite accurate. >>> >>> After our discussion yesterday, does that still hold true? >> >> No. >> >>>> IIUC in __kvm_mmu_max_mapping_level(), we skip considering >>>> host_pfn_mapping_level() if the gfn is private because private memory >>>> will not be mapped to userspace, so there's no need to query userspace >>>> page tables in host_pfn_mapping_level(). >>> >>> I think the reason was that: for private we won't be walking the user space >>> pages tables. >>> >>> Once guest_memfd is also responsible for the shared part, why should this >>> here still be private-only, and why should we consider querying a user space >>> mapping that might not even exist? >> >> +1, one of the big selling points for guest_memfd beyond CoCo is that it provides >> guest-first memory. It is very explicitly an intended feature that the guest >> mappings KVM creates can be a superset of the host userspace mappings. E.g. the >> guest can use larger page sizes, have RW while the host has RO, etc. > > Do you mean that __kvm_mmu_max_mapping_level() should, in addition to > the parameter renaming from is_private to is_gmem, do something like > > if (is_gmem) > return kvm_gmem_get_max_mapping_level(slot, gfn); I assume you mean, not looking at lpage_info at all? I have limited understanding what lpage_info is or what it does. I believe all it adds is a mechanism to *disable* large page mappings. We want to disable large pages if (using 2M region as example) (a) Mixed memory attributes. If a PFN falls into a 2M region, and parts of that region are shared vs. private (mixed memory attributes -> KVM_LPAGE_MIXED_FLAG) -> With gmem-shared we could have mixed memory attributes, not a PFN fracturing. (PFNs don't depend on memory attributes) (b) page track: intercepting (mostly write) access to GFNs So, I wonder if we still have to take care of lpage_info, at least for handling (b) correctly [I assume so]. Regarding (a) I am not sure: once memory attributes are handled by gmem in the gmem-shared case. IIRC, with AMD SEV we might still have to honor it? But gmem itself could handle that. What we could definitely do here for now is: if (is_gmem) /* gmem only supports 4k pages for now. */ return PG_LEVEL_4K; And not worry about lpage_infor for the time being, until we actually do support larger pages. > > and basically defer to gmem as long as gmem should be used for this gfn? > > There is another call to __kvm_mmu_max_mapping_level() via > kvm_mmu_max_mapping_level() beginning from recover_huge_pages_range(), > and IIUC that doesn't go through guest_memfd. > > Hence, unlike the call to __kvm_mmu_max_mapping_level() from the KVM x86 > MMU fault path, guest_memfd didn't get a chance to provide its input in > the form of returning max_order from kvm_gmem_get_pfn(). Right, we essentially say that "this is a private fault", likely assuming that we already verified earlier that the memory is also private. [I can see that happening when the function is called through direct_page_fault()] We could simply call kvm_mmu_max_mapping_level() from kvm_mmu_hugepage_adjust() I guess. (could possibly be optimized later) -- Cheers, David / dhildenb