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[2003:d8:2f25:9c00:e2c7:6eb5:8a51:1c60]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-447f6ffaa6esm71798335e9.16.2025.05.21.06.45.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 21 May 2025 06:45:04 -0700 (PDT) Message-ID: Date: Wed, 21 May 2025 15:45:01 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 14/17] KVM: arm64: Enable mapping guest_memfd in arm64 To: Fuad Tabba Cc: 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, seanjc@google.com, 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, ackerleytng@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, ira.weiny@intel.com References: <20250513163438.3942405-1-tabba@google.com> <20250513163438.3942405-15-tabba@google.com> <2084504e-2a11-404a-bbe8-930384106f53@redhat.com> <5da72da7-b82c-4d70-ac86-3710a046b836@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 21.05.25 15:32, Fuad Tabba wrote: > Hi David, > > On Wed, 21 May 2025 at 14:22, David Hildenbrand wrote: >> >> On 21.05.25 15:15, Fuad Tabba wrote: >>> Hi David, >>> >>> On Wed, 21 May 2025 at 13:44, David Hildenbrand wrote: >>>> >>>> On 21.05.25 12:29, Fuad Tabba wrote: >>>>> On Wed, 21 May 2025 at 11:26, David Hildenbrand wrote: >>>>>> >>>>>> On 21.05.25 12:12, Fuad Tabba wrote: >>>>>>> Hi David, >>>>>>> >>>>>>> On Wed, 21 May 2025 at 09:05, David Hildenbrand wrote: >>>>>>>> >>>>>>>> On 13.05.25 18:34, Fuad Tabba wrote: >>>>>>>>> Enable mapping guest_memfd in arm64. For now, it applies to all >>>>>>>>> VMs in arm64 that use guest_memfd. In the future, new VM types >>>>>>>>> can restrict this via kvm_arch_gmem_supports_shared_mem(). >>>>>>>>> >>>>>>>>> Signed-off-by: Fuad Tabba >>>>>>>>> --- >>>>>>>>> arch/arm64/include/asm/kvm_host.h | 10 ++++++++++ >>>>>>>>> arch/arm64/kvm/Kconfig | 1 + >>>>>>>>> 2 files changed, 11 insertions(+) >>>>>>>>> >>>>>>>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h >>>>>>>>> index 08ba91e6fb03..2514779f5131 100644 >>>>>>>>> --- a/arch/arm64/include/asm/kvm_host.h >>>>>>>>> +++ b/arch/arm64/include/asm/kvm_host.h >>>>>>>>> @@ -1593,4 +1593,14 @@ static inline bool kvm_arch_has_irq_bypass(void) >>>>>>>>> return true; >>>>>>>>> } >>>>>>>>> >>>>>>>>> +static inline bool kvm_arch_supports_gmem(struct kvm *kvm) >>>>>>>>> +{ >>>>>>>>> + return IS_ENABLED(CONFIG_KVM_GMEM); >>>>>>>>> +} >>>>>>>>> + >>>>>>>>> +static inline bool kvm_arch_vm_supports_gmem_shared_mem(struct kvm *kvm) >>>>>>>>> +{ >>>>>>>>> + return IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM); >>>>>>>>> +} >>>>>>>>> + >>>>>>>>> #endif /* __ARM64_KVM_HOST_H__ */ >>>>>>>>> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig >>>>>>>>> index 096e45acadb2..8c1e1964b46a 100644 >>>>>>>>> --- a/arch/arm64/kvm/Kconfig >>>>>>>>> +++ b/arch/arm64/kvm/Kconfig >>>>>>>>> @@ -38,6 +38,7 @@ menuconfig KVM >>>>>>>>> select HAVE_KVM_VCPU_RUN_PID_CHANGE >>>>>>>>> select SCHED_INFO >>>>>>>>> select GUEST_PERF_EVENTS if PERF_EVENTS >>>>>>>>> + select KVM_GMEM_SHARED_MEM >>>>>>>>> help >>>>>>>>> Support hosting virtualized guest machines. >>>>>>>>> >>>>>>>> >>>>>>>> Do we have to reject somewhere if we are given a guest_memfd that was >>>>>>>> *not* created using the SHARED flag? Or will existing checks already >>>>>>>> reject that? >>>>>>> >>>>>>> We don't reject, but I don't think we need to. A user can create a >>>>>>> guest_memfd that's private in arm64, it would just be useless. >>>>>> >>>>>> But the arm64 fault routine would not be able to handle that properly, no? >>>>> >>>>> Actually it would. The function user_mem_abort() doesn't care whether >>>>> it's private or shared. It would fault it into the guest correctly >>>>> regardless. >>>> >>>> >>>> I think what I meant is that: if it's !shared (private only), shared >>>> accesses (IOW all access without CoCo) should be taken from the user >>>> space mapping. >>>> >>>> But user_mem_abort() would blindly go to kvm_gmem_get_pfn() because >>>> "is_gmem = kvm_slot_has_gmem(memslot) = true". >>> >>> Yes, since it is a gmem-backed slot. >>> >>>> In other words, arm64 would have to *ignore* guest_memfd that does not >>>> support shared? >>>> >>>> That's why I was wondering whether we should just immediately refuse >>>> such guest_memfds. >>> >>> My thinking is that if a user deliberately creates a >>> guest_memfd-backed slot without designating it as being sharable, then >>> either they would find out when they try to map that memory to the >>> host userspace (mapping it would fail), or it could be that they >>> deliberately want to set up a VM with memslots that not mappable at >>> all by the host. >> >> Hm. But that would meant that we interpret "private" memory as a concept >> that is not understood by the VM. Because the VM does not know what >> "private" memory is ... >> >>> Perhaps to add some layer of security (although a >>> very flimsy one, since it's not a confidential guest). >> >> Exactly my point. If you don't want to mmap it then ... don't mmap it :) >> >>> >>> I'm happy to a check to prevent this. The question is, how to do it >>> exactly (I assume it would be in kvm_gmem_create())? Would it be >>> arch-specific, i.e., prevent arm64 from creating non-shared >>> guest_memfd backed memslots? Or do it by VM type? Even if we do it by >>> VM-type it would need to be arch-specific, since we allow private >>> guest_memfd slots for the default VM in x86, but we wouldn't for >>> arm64. >>> >>> We could add another function, along the lines of >>> kvm_arch_supports_gmem_only_shared_mem(), but considering that it >>> actually works, and (arguably) would behave as intended, I'm not sure >>> if it's worth the complexity. >>> >>> What do you think? >> >> My thinking was to either block this at slot creation time or at >> guest_memfd creation time. And we should probably block that for other >> VM types as well that do not support private memory? >> >> I mean, creating guest_memfd for private memory when there is no concept >> of private memory for the VM is ... weird, no? :) > > Actually, I could add this as an arch-specific check in > arch/arm64/kvm/mmu.c:kvm_arch_prepare_memory_region(). That way, core > KVM/guest_memfd code doesn't need to handle this arm64-specific behavior. > > Does that sound good? Yes, but only do so if you agree. -- Cheers, David / dhildenb