On Sun, Aug 16, 2026, Ackerley Tng wrote: > Sean Christopherson <[email protected]> writes: > > > On Thu, Aug 13, 2026, Ackerley Tng wrote: > >> Sean Christopherson <[email protected]> writes: > >> > That's why I think it's worth analyzing the cost: if it's in the > >> > noise, leave it alone. If it's meaningful, figure out a not-too-gross > >> > way to skip > >> > the entire thing if kvm_arch_gmem_make_shared() is a glorified nop in > >> > the end. > >> > >> Is noise defined relative to the entire conversion process? Would this > >> benchmark look like > >> > >> 1. Convert 4G to shared on TDX with CONFIG_AMD_SEV defined > >> 2. Convert 4G to shared on TDX without CONFIG_AMD_SEV defined > >> > >> and then compare the difference in time taken? > > > > That'd work, though I was envisioning something even simpler: use rdtsc() to > > count the cycles it takes to iterate over various ranges of memory. Do > > whatever > > is easiest for you though. > > I made some changes to add rdtsc() for the conversion process as Sean > suggested [1], and exercised conversion like this [2]: > > 1. Initialize some memory as private > 2. Get the guest to fault them into Secure EPTs > 3. Converts the memory to shared <<== this is being benchmarked > 4. Converts memory back to private > > I made it build the VM once and convert 5 times: > > ./gmem_benchmark_tdx_convert --iterations=5 --size=1g
... > And here's the above, tabulated: > > nr_pages make_shared total percentage > ---------- --------------- --------------- ------------ > 1 930 39278 2.3677% > 1 252 28060 0.8981% > 1 176 26952 0.6530% > 1 176 27038 0.6509% > 1 176 26980 0.6523% > 1 1072 37236 2.8789% > 1 316 28338 1.1151% > 1 176 27182 0.6475% > 1 176 26972 0.6525% > 1 176 26886 0.6546% > 262144 15041018 6616067680 0.2273% > 262144 14937462 6608542680 0.2260% > 262144 15138858 6599494898 0.2294% > 262144 15721972 6610219850 0.2378% > 262144 15000406 6615114540 0.2268% > 1048576 61902982 26400884028 0.2345% > 1048576 61746114 26401170984 0.2339% > 1048576 61096794 26404409058 0.2314% > 1048576 61446290 26447461896 0.2323% > 1048576 61774646 26444608360 0.2336% > > Looks to me it is within noise. > > I also actually tried measuring the conversion time from userspace with > CONFIG_AMD_SEV enabled and disabled. Converting a 1G-sized TD was faster > by 0.2%, which is in line with the above table. Interestingly, when > converting a 4G-sized TD, skipping kvm_gmem_make_shared() was _slower_ > over 2 runs. I don't have an explanation for that. Might be some cache/memory locality benefits? Though with a conversion that big, it could also be nothing more than bad luck. > I think the code was correct. (If it makes a difference, I skipped > kvm_gmem_make_shared() using a custom guest_memfd creation time flag and > skipped make_shared if the flag was set on the inode.) > > I thought adding a kvm_arch_has_gmem_make_shared(), defaulting it to I would do kvm_arch_has_gmem_convert() for consistency with the Kconfigs, and because the cost of the reclaim invocation is a non-issue. > false for all archs and having x86 override with > !!kvm_x86_ops.gmem_make_shared is not too bad either: > + doesn't leak anything, since the function being called is > kvm_arch_gmem_make_shared and the accompanying function is > kvm_arch_has_gmem_make_shared. Or maybe just a little, since all the > other ops don't have the accompanying _has_ function > + it's a kernel-internal thing > + not too many lines of code, not too complex It also provides a good excuse to kill off the #idfefs in guest_memfd.c. Compile tested only, but I'm thinking this? From: Sean Christopherson <[email protected]> Date: Mon, 17 Aug 2026 12:31:50 -0700 Subject: [PATCH] KVM: guest_memfd: Optimize away conversion overheads via dead-code elimination Add and use kvm_arch_has_gmem_convert() to guard guest_memfd's invocation of arch hooks related to converting memory between private and shared, as only one half of the x86 CoCo duo needs the runtime hooks (any pre-work is pure overhead for TDX). At this exact moment, the overhead is negligible, but that will change when in-place conversion comes along, at which point to-shared conversions will "need" to find all affected folios prior to calling into arch code. In quotes because very technically that work could be pushed to arch code, but that would bleed guest_memfd details into arch code and would be far worse than adding yet another kvm_arch_has... hook. Opportunistically provide the kvm_arch_gmem_make_private() declaration, and rely on dead-code elimination to eliminate the call to non-existent code when CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT=n. Reported-by: Binbin Wu <[email protected]> Closes: https://lore.kernel.org/all/[email protected] Suggested-by: Ackerley Tng <[email protected]> Signed-off-by: Sean Christopherson <[email protected]> --- arch/x86/include/asm/kvm_host.h | 3 +++ include/linux/kvm_host.h | 3 ++- virt/kvm/guest_memfd.c | 5 ++--- 3 files changed, 7 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 283847619ff8..5d5a7723abb6 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -1854,6 +1854,9 @@ enum kvm_intr_type { #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES #define kvm_arch_has_private_mem(kvm) ((kvm)->arch.has_private_mem) #endif +#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT +#define kvm_arch_has_gmem_convert() (!!kvm_x86_ops.gmem_make_private) +#endif #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state) diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 03bfc92864b6..e824ba59c60c 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -2599,9 +2599,10 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm, } #endif /* CONFIG_KVM_GUEST_MEMFD */ -#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT int kvm_arch_gmem_make_private(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, kvm_pfn_t nr_pages); +#ifndef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT +#define kvm_arch_has_gmem_convert() false #endif #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_POPULATE diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index b596486d184c..39d94938b5f6 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -773,11 +773,10 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot, folio_mark_uptodate(folio); } -#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT - if (kvm_gmem_is_private_mem(file_inode(file), index)) + if (kvm_arch_has_gmem_convert() && + kvm_gmem_is_private_mem(file_inode(file), index)) r = kvm_arch_gmem_make_private(kvm, gfn, *pfn, (kvm_pfn_t)1 << *max_order); -#endif folio_unlock(folio); base-commit: 1b731e5ded480bd1e5546aed35584238661ce72e --
