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Fri, 11 Jul 2025 04:09:55 -0700 (PDT) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20250709105946.4009897-1-tabba@google.com> <20250709105946.4009897-15-tabba@google.com> <6fc1edd4-8a3c-4cba-8779-461a16b5126e@redhat.com> In-Reply-To: <6fc1edd4-8a3c-4cba-8779-461a16b5126e@redhat.com> From: Fuad Tabba Date: Fri, 11 Jul 2025 12:09:19 +0100 X-Gm-Features: Ac12FXzb-fN6GfwAX5mOcl0uP18BvI0qeajeteMMtilzL-SP9ogjqoPftjSQs3k Message-ID: Subject: Re: [PATCH v13 14/20] KVM: x86: Enable guest_memfd mmap for default VM type To: David Hildenbrand Cc: kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, kvmarm@lists.linux.dev, 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 Content-Type: text/plain; charset="UTF-8" Hi David, On Fri, 11 Jul 2025 at 10:45, David Hildenbrand wrote: > > On 09.07.25 12:59, Fuad Tabba wrote: > > Enable host userspace mmap support for guest_memfd-backed memory when > > running KVM with the KVM_X86_DEFAULT_VM type: > > > > * Define kvm_arch_supports_gmem_mmap() for KVM_X86_DEFAULT_VM: Introduce > > the architecture-specific kvm_arch_supports_gmem_mmap() macro, > > specifically enabling mmap support for KVM_X86_DEFAULT_VM instances. > > This macro, gated by CONFIG_KVM_GMEM_SUPPORTS_MMAP, ensures that only > > the default VM type can leverage guest_memfd mmap functionality on > > x86. This explicit enablement prevents CoCo VMs, which use guest_memfd > > primarily for private memory and rely on hardware-enforced privacy, > > from accidentally exposing guest memory via host userspace mappings. > > > > * Select CONFIG_KVM_GMEM_SUPPORTS_MMAP in KVM_X86: Enable the > > CONFIG_KVM_GMEM_SUPPORTS_MMAP Kconfig option when KVM_X86 is selected. > > This ensures that the necessary code for guest_memfd mmap support > > (introduced earlier) is compiled into the kernel for x86. This Kconfig > > option acts as a system-wide gate for the guest_memfd mmap capability. > > It implicitly enables CONFIG_KVM_GMEM, making guest_memfd available, > > and then layers the mmap capability on top specifically for the > > default VM. > > > > These changes make guest_memfd a more versatile memory backing for > > standard KVM guests, allowing VMMs to use a unified guest_memfd model > > for both private (CoCo) and non-private (default) VMs. This is a > > prerequisite for use cases such as running Firecracker guests entirely > > backed by guest_memfd and implementing direct map removal for non-CoCo > > VMs. > > > > Co-developed-by: Ackerley Tng > > Signed-off-by: Ackerley Tng > > Signed-off-by: Fuad Tabba > > --- > > arch/x86/include/asm/kvm_host.h | 9 +++++++++ > > arch/x86/kvm/Kconfig | 1 + > > arch/x86/kvm/x86.c | 3 ++- > > 3 files changed, 12 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > > index 4c764faa12f3..4c89feaa1910 100644 > > --- a/arch/x86/include/asm/kvm_host.h > > +++ b/arch/x86/include/asm/kvm_host.h > > @@ -2273,9 +2273,18 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level, > > #ifdef CONFIG_KVM_GMEM > > #define kvm_arch_has_private_mem(kvm) ((kvm)->arch.has_private_mem) > > #define kvm_arch_supports_gmem(kvm) ((kvm)->arch.supports_gmem) > > + > > +/* > > + * CoCo VMs with hardware support that use guest_memfd only for backing private > > + * memory, e.g., TDX, cannot use guest_memfd with userspace mapping enabled. > > + */ > > +#define kvm_arch_supports_gmem_mmap(kvm) \ > > + (IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP) && \ > > + (kvm)->arch.vm_type == KVM_X86_DEFAULT_VM) > > #else > > #define kvm_arch_has_private_mem(kvm) false > > #define kvm_arch_supports_gmem(kvm) false > > +#define kvm_arch_supports_gmem_mmap(kvm) false > > #endif > > > > #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state) > > diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig > > index df1fdbb4024b..239637b663dc 100644 > > --- a/arch/x86/kvm/Kconfig > > +++ b/arch/x86/kvm/Kconfig > > @@ -47,6 +47,7 @@ config KVM_X86 > > select KVM_GENERIC_HARDWARE_ENABLING > > select KVM_GENERIC_PRE_FAULT_MEMORY > > select KVM_GENERIC_GMEM_POPULATE if KVM_SW_PROTECTED_VM > > + select KVM_GMEM_SUPPORTS_MMAP > > select KVM_WERROR if WERROR > > Given the error, likely we want to limit to 64BIT. > > select KVM_GMEM_SUPPORTS_MMAP if X86_64 Will do. Cheers, /fuad > -- > Cheers, > > David / dhildenb >