On Thu, Oct 6, 2022 at 1:49 AM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote: > > Vipin Sharma <vipinsh@xxxxxxxxxx> writes: > > > Hi Vitaly, Yury, Sunil, Tianyu > > Hi Vipin! > > > > > Before I work on a patch series and send it out to the KVM mailing > > list, I wanted to check with you a potential Windows VM optimization > > and see if you have worked on it or if you know about some obvious > > known blockers regarding this feature. > > > > Hypervisor Top-Level Functional Specification v6.0b mentions a hypercall: > > > > HvExtCallGetBootZeroedMemory > > Call Code = 0x8002 > > > > This hypercall can be used by Windows guest to know which pages are > > already zeroed and then guest can avoid zeroing them again during the > > boot, resulting in Windows VM faster boot time and less memory usage. > > > > KVM currently doesn't implement this feature. I am thinking of > > implementing it, here is a rough code flow: > > 1. KVM will set bit 20 in EBX of CPUID leaf 0x40000003 to let the > > Windows guest know that it can use the extended hypercall interface. > > 2. Guest during the boot will use hypercall HvExtCallQueryCapabilities > > (Call Code = 0x8001) to see which extended calls are available. > > 3. KVM will respond to guest that the hypercall > > HvExtCallGetBootZeroedMemory is available. > > 4. Guest will issue the hypercall HvExtCallGetBootZeroedMemory to know > > which pages are zeroed. > > 5. KVM or userspace VMM will respond with GPA and page count to guest. > > I think it's VMM's responsibility. How would KVM know if the memory > allocated to the guest was zeroed or not? > > The easiest solution would be to just pass through this hypercall to the > VMM and let it respond. Alternatively, we can probably add a flag to > KVM_SET_USER_MEMORY_REGION to either indicate that the memory is zeroed > or to actually ask KVM to zero it. This way we will have the required > information in KVM. I'm not sure if it's worth it, Windows probably > calls HvExtCallGetBootZeroedMemory just once upon boot so handling it in > the VMM is totally fine. I agree with you. Since, it is probably only upon boot time, handling in VMM should be okay. > > > 6. Guest will skip zeroing these pages, resulting in faster boot and > > less memory utilization of guest. > > > > This seems like a very easy win for KVM to increase Windows guest boot > > performance but I am not sure if I am overlooking something. If you > > are aware of any potential side effects of enabling these hypercalls > > or some other issue I am not thinking about please let me know, > > otherwise, I can start working on this feature and send RFC patches to > > the mailing list. > > I dug through my git archives and found that I've actually tried > HvExtCallQueryCapabilities back in 2018 but for some reason Windows > versions I was testing didn't use it (hope it wasn't some silly mistake > like forgotten CPUID bit on my part :-) so I put it aside and never got > back to it. Thanks for picking this up! Keeping my fingers crossed! Thanks for the feedback. > > -- > Vitaly >
