On Fri, 10 Apr 2026 16:19:24 +0100, "Kalyazin, Nikita" <[email protected]> wrote: > > From: Patrick Roy <[email protected]> > > Support for GUEST_MEMFD_FLAG_NO_DIRECT_MAP on arm64 depends on 1) direct > map manipulations at 4k granularity being possible, and 2) FEAT_S2FWB. > > 1) is met whenever the direct map is set up at 4k granularity (e.g. not > with huge/gigantic pages) at boottime, as due to ARM's > break-before-make semantics, breaking huge mappings into 4k mappings in > the direct map is not possible (BBM would require temporary invalidation > of the entire huge mapping, even if only a 4k subrange should be zapped, > which will probably crash the kernel). However, the current default for > rodata_full is true, which forces a 4k direct map.
Where is this 4kB requirement enforced? Or is it that you means "PAGE_SIZE"? > > 2) is required to allow KVM to elide cache coherency operations when > installing stage 2 page tables, which require the direct map to be > entry for the newly mapped memory to be present (which it will not be, > as guest_memfd would have removed direct map entries in > kvm_gmem_get_pfn()). > > Cc: Will Deacon <[email protected]> > Signed-off-by: Patrick Roy <[email protected]> > Reviewed-by: David Hildenbrand (Arm) <[email protected]> > Signed-off-by: Nikita Kalyazin <[email protected]> > --- > arch/arm64/include/asm/kvm_host.h | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/arch/arm64/include/asm/kvm_host.h > b/arch/arm64/include/asm/kvm_host.h > index 70cb9cfd760a..fbdd43e7e94e 100644 > --- a/arch/arm64/include/asm/kvm_host.h > +++ b/arch/arm64/include/asm/kvm_host.h > @@ -19,6 +19,7 @@ > #include <linux/maple_tree.h> > #include <linux/percpu.h> > #include <linux/psci.h> > +#include <linux/set_memory.h> > #include <asm/arch_gicv3.h> > #include <asm/barrier.h> > #include <asm/cpufeature.h> > @@ -1682,6 +1683,18 @@ static __always_inline enum fgt_group_id > __fgt_reg_to_group_id(enum vcpu_sysreg > \ > p; \ > }) > +#ifdef CONFIG_KVM_GUEST_MEMFD KVM/arm64 has: select KVM_GUEST_MEMFD > +static inline bool kvm_arch_gmem_supports_no_direct_map(struct kvm *kvm) > +{ > + /* > + * Without FWB, direct map access is needed in kvm_pgtable_stage2_map(), > + * as it calls dcache_clean_inval_poc(). > + */ > + return can_set_direct_map() && > cpus_have_final_cap(ARM64_HAS_STAGE2_FWB); > +} > +#define kvm_arch_gmem_supports_no_direct_map > kvm_arch_gmem_supports_no_direct_map > +#endif /* CONFIG_KVM_GUEST_MEMFD */ Why is it an inline function? Given that it takes a kvm parameter, I fully expect that you'll have to evaluate this at some point. But since struct kvm cannot be dereferenced in asm/kvm_host.h, it will have to move. How about doing it right now? M. -- Without deviation from the norm, progress is not possible.
