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"shuah@kernel.org" , "song@kernel.org" , "surenb@google.com" , "suzuki.poulose@arm.com" , "svens@linux.ibm.com" , "tglx@linutronix.de" , "trondmy@kernel.org" , "vbabka@suse.cz" , "viro@zeniv.linux.org.uk" , "weixugc@google.com" , "will@kernel.org" , "willy@infradead.org" , "x86@kernel.org" , "Cali, Marco" , "yonghong.song@linux.dev" , "yuanchu@google.com" , "yuzenghui@huawei.com" , "zhengqi.arch@bytedance.com" Subject: Re: [PATCH v6 05/11] KVM: guest_memfd: Add flag to remove from direct map Thread-Topic: [PATCH v6 05/11] KVM: guest_memfd: Add flag to remove from direct map Thread-Index: AQHcKT757eD2hLtXdESX+jHFR62e2Q== Date: Fri, 19 Sep 2025 08:25:36 +0000 Message-ID: <20250919082534.17376-1-roypat@amazon.co.uk> References: In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [172.19.88.180] Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-Rspam-User: 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Mike,=0A= =0A= ...=0A= =0A= >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/= kvm_host.h=0A= >> index 2f2394cce24e..0bfd8e5fd9de 100644=0A= >> --- a/arch/arm64/include/asm/kvm_host.h=0A= >> +++ b/arch/arm64/include/asm/kvm_host.h=0A= >> @@ -19,6 +19,7 @@=0A= >> #include =0A= >> #include =0A= >> #include =0A= >> +#include =0A= >> #include =0A= >> #include =0A= >> #include =0A= >> @@ -1706,5 +1707,16 @@ void compute_fgu(struct kvm *kvm, enum fgt_group_= id fgt);=0A= >> void get_reg_fixed_bits(struct kvm *kvm, enum vcpu_sysreg reg, u64 *res= 0, u64 *res1);=0A= >> void check_feature_map(void);=0A= >>=0A= >> +#ifdef CONFIG_KVM_GUEST_MEMFD=0A= >> +static inline bool kvm_arch_gmem_supports_no_direct_map(void)=0A= >> +{=0A= >> + /*=0A= >> + * Without FWB, direct map access is needed in kvm_pgtable_stage2_= map(),=0A= >> + * as it calls dcache_clean_inval_poc().=0A= >> + */=0A= >> + return can_set_direct_map() && cpus_have_final_cap(ARM64_HAS_STAGE= 2_FWB);=0A= >> +}=0A= >> +#define kvm_arch_gmem_supports_no_direct_map kvm_arch_gmem_supports_no_= direct_map=0A= >> +#endif /* CONFIG_KVM_GUEST_MEMFD */=0A= >>=0A= >> #endif /* __ARM64_KVM_HOST_H__ */=0A= >> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h=0A= >> index 1d0585616aa3..a9468bce55f2 100644=0A= >> --- a/include/linux/kvm_host.h=0A= >> +++ b/include/linux/kvm_host.h=0A= >> @@ -36,6 +36,7 @@=0A= >> #include =0A= >> #include =0A= >> #include =0A= >> +#include =0A= > =0A= > The set_memory APIs are not used in the header, no need to include it her= e.=0A= > =0A= =0A= Ack!=0A= =0A= >> #include =0A= >> #include =0A= >> @@ -731,6 +732,12 @@ static inline bool kvm_arch_has_private_mem(struct = kvm *kvm)=0A= >> bool kvm_arch_supports_gmem_mmap(struct kvm *kvm);=0A= >> #endif=0A= >>=0A= >> +#ifdef CONFIG_KVM_GUEST_MEMFD=0A= >> +#ifndef kvm_arch_gmem_supports_no_direct_map=0A= >> +#define kvm_arch_gmem_supports_no_direct_map can_set_direct_map=0A= >> +#endif=0A= >> +#endif /* CONFIG_KVM_GUEST_MEMFD */=0A= >> +=0A= >> #ifndef kvm_arch_has_readonly_mem=0A= >> static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)=0A= >> {=0A= >> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h=0A= >> index 6efa98a57ec1..33c8e8946019 100644=0A= >> --- a/include/uapi/linux/kvm.h=0A= >> +++ b/include/uapi/linux/kvm.h=0A= >> @@ -963,6 +963,7 @@ struct kvm_enable_cap {=0A= >> #define KVM_CAP_RISCV_MP_STATE_RESET 242=0A= >> #define KVM_CAP_ARM_CACHEABLE_PFNMAP_SUPPORTED 243=0A= >> #define KVM_CAP_GUEST_MEMFD_MMAP 244=0A= >> +#define KVM_CAP_GUEST_MEMFD_NO_DIRECT_MAP 245=0A= >>=0A= >> struct kvm_irq_routing_irqchip {=0A= >> __u32 irqchip;=0A= >> @@ -1600,6 +1601,7 @@ struct kvm_memory_attributes {=0A= >>=0A= >> #define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_cre= ate_guest_memfd)=0A= >> #define GUEST_MEMFD_FLAG_MMAP (1ULL << 0)=0A= >> +#define GUEST_MEMFD_FLAG_NO_DIRECT_MAP (1ULL << 1)=0A= >>=0A= >> struct kvm_create_guest_memfd {=0A= >> __u64 size;=0A= >> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c=0A= >> index 81028984ff89..3c64099fc98a 100644=0A= >> --- a/virt/kvm/guest_memfd.c=0A= >> +++ b/virt/kvm/guest_memfd.c=0A= >> @@ -4,6 +4,7 @@=0A= >> #include =0A= >> #include =0A= >> #include =0A= >> +#include =0A= >>=0A= >> #include "kvm_mm.h"=0A= >>=0A= >> @@ -42,9 +43,24 @@ static int __kvm_gmem_prepare_folio(struct kvm *kvm, = struct kvm_memory_slot *slo=0A= >> return 0;=0A= >> }=0A= >>=0A= >> -static inline void kvm_gmem_mark_prepared(struct folio *folio)=0A= >> +static bool kvm_gmem_test_no_direct_map(struct inode *inode)=0A= >> {=0A= >> - folio_mark_uptodate(folio);=0A= >> + return ((unsigned long) inode->i_private) & GUEST_MEMFD_FLAG_NO_DI= RECT_MAP;=0A= >> +}=0A= >> +=0A= >> +static inline int kvm_gmem_mark_prepared(struct folio *folio)=0A= >> +{=0A= >> + struct inode *inode =3D folio_inode(folio);=0A= >> + int r =3D 0;=0A= >> +=0A= >> + if (kvm_gmem_test_no_direct_map(inode))=0A= >> + r =3D set_direct_map_valid_noflush(folio_page(folio, 0), f= olio_nr_pages(folio),=0A= >> + false);=0A= >> +=0A= >> + if (!r)=0A= >> + folio_mark_uptodate(folio);=0A= >> +=0A= >> + return r;=0A= >> }=0A= >>=0A= >> /*=0A= >> @@ -82,7 +98,7 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, str= uct kvm_memory_slot *slot,=0A= >> index =3D ALIGN_DOWN(index, 1 << folio_order(folio));=0A= >> r =3D __kvm_gmem_prepare_folio(kvm, slot, index, folio);=0A= >> if (!r)=0A= >> - kvm_gmem_mark_prepared(folio);=0A= >> + r =3D kvm_gmem_mark_prepared(folio);=0A= > =0A= > If this fails, shouldn't we undo __kvm_gmem_prepare_folio()?=0A= >=0A= =0A= Yes, good point. I'm not sure if we can undo preparation (its only used=0A= by AMD-SEV right now, for passing off the page to the CoCo context). But=0A= not undoing it means that guest_memfd will consider the page unprepared,=0A= and zero it again the next time it's accesses, which will cause a=0A= machine check because the page has already been passed off to the=0A= confidential world.=0A= =0A= We talked about this in the guest_memfd upstream call yesterday, and=0A= decided that in addition to this problem, we want to separate=0A= preparedness tracking from direct map removal state tracking anyway (and=0A= move preparedness tracking outside of guest_memfd into the arch specific=0A= code). And if direct map state and preparedness are separate bits, then=0A= we can accurately record the state of "preparation worked but direct map=0A= removal failed".=0A= =0A= >>=0A= >> return r;=0A= >> }=0A= >> @@ -344,8 +360,15 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struc= t vm_fault *vmf)=0A= >> }=0A= >>=0A= >> if (!folio_test_uptodate(folio)) {=0A= >> + int err =3D 0;=0A= >> +=0A= >> clear_highpage(folio_page(folio, 0));=0A= >> - kvm_gmem_mark_prepared(folio);=0A= >> + err =3D kvm_gmem_mark_prepared(folio);=0A= >> +=0A= >> + if (err) {=0A= >> + ret =3D vmf_error(err);=0A= >> + goto out_folio;=0A= >> + }=0A= >> }=0A= >>=0A= >> vmf->page =3D folio_file_page(folio, vmf->pgoff);=0A= >> @@ -436,6 +459,16 @@ static void kvm_gmem_free_folio(struct address_spac= e *mapping,=0A= >> kvm_pfn_t pfn =3D page_to_pfn(page);=0A= >> int order =3D folio_order(folio);=0A= >>=0A= >> + /*=0A= >> + * Direct map restoration cannot fail, as the only error condition= =0A= >> + * for direct map manipulation is failure to allocate page tables= =0A= >> + * when splitting huge pages, but this split would have already=0A= >> + * happened in set_direct_map_invalid_noflush() in kvm_gmem_mark_p= repared().=0A= >> + * Thus set_direct_map_valid_noflush() here only updates prot bits= .=0A= >> + */=0A= >> + if (kvm_gmem_test_no_direct_map(mapping->host))=0A= >> + set_direct_map_valid_noflush(page, folio_nr_pages(folio), = true);=0A= >> +=0A= >> kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order));=0A= >> }=0A= >>=0A= >> @@ -500,6 +533,9 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t= size, u64 flags)=0A= >> /* Unmovable mappings are supposed to be marked unevictable as wel= l. */=0A= >> WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));=0A= >>=0A= >> + if (flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)=0A= >> + mapping_set_no_direct_map(inode->i_mapping);=0A= >> +=0A= >> kvm_get_kvm(kvm);=0A= >> gmem->kvm =3D kvm;=0A= >> xa_init(&gmem->bindings);=0A= >> @@ -524,6 +560,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_crea= te_guest_memfd *args)=0A= >> if (kvm_arch_supports_gmem_mmap(kvm))=0A= >> valid_flags |=3D GUEST_MEMFD_FLAG_MMAP;=0A= >>=0A= >> + if (kvm_arch_gmem_supports_no_direct_map())=0A= >> + valid_flags |=3D GUEST_MEMFD_FLAG_NO_DIRECT_MAP;=0A= >> +=0A= >> if (flags & ~valid_flags)=0A= >> return -EINVAL;=0A= >>=0A= >> @@ -768,7 +807,7 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_= gfn, void __user *src, long=0A= >> p =3D src ? src + i * PAGE_SIZE : NULL;=0A= >> ret =3D post_populate(kvm, gfn, pfn, p, max_order, opaque)= ;=0A= >> if (!ret)=0A= >> - kvm_gmem_mark_prepared(folio);=0A= >> + ret =3D kvm_gmem_mark_prepared(folio);=0A= >>=0A= >> put_folio_and_exit:=0A= >> folio_put(folio);=0A= ...=0A= =0A= >=0A= > Sincerely yours,=0A= > Mike.=0A= Best, =0A= Patrick=0A= =0A=