Re: [PATCH v10 12/41] KVM: guest_memfd: Call arch make_shared callback for to-shared conversion
Sean Christopherson <[email protected]>
| Newsgroups | org.kernel.vger.linux-kselftest,dev.linux.lists.linux-coco,org.kernel.vger.kvm,org.kernel.vger.linux-doc,org.kernel.vger.linux-kernel,org.kernel.vger.linux-trace-kernel,org.kvack.linux-mm |
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| Message-ID | <[email protected]> |
On Mon, Aug 17, 2026, Sean Christopherson wrote: > 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. Forgot to say "why" I think it's worth adding kvm_arch_has_gmem_convert(): it's a trivial change in the grand scheme, and I can see the ~1000 cycles of overhead being problematic in use cases that do many one-off conversions, e.g. for guests whose shared memory is scattered all over.