[pve-devel] applied: [PATCH v2 qemu-server] fix #2083: Add hv_tlbflush, hv_ipi, hv_evmcs enlightenments

[Thomas Lamprecht]

On 6/19/19 10:23 AM, Stefan Reiter wrote:
> Kernels 4.18+ (4.17+ for evmcs) support new Hyper-V enlightenments for
> Windows KVM guests. QEMU supports these since 3.0 and 3.1 respectively.
> tlbflush and ipi improve performance on overcommitted systems, evmcs
> improves nested virtualization.
> 
> It's not entirely clear to me if Win7 already supports these, but since
> it doesn't cause any performance penalties (and it works fine without
> crashing, which makes sense either way, because Hyper-V enlightenments
> are opt-in by the guest OS), enabling it regardless should be fine.
> (As opposed to adding a new if branch for win8+)
> 
> Feature explanations to the best of my understanding:
> 
> hv_tlbflush allows the guest OS to trigger tlb shootdowns via a
> hypercall. This allows CPUs to be identified via their vpindex (which
> makes hv_vpindex a prerequisite to hv_tlbflush, but that is already
> handled in our code). In overcommited configurations, where multiple
> vCPUs reside on one pCPU, this increases performance of guest tlb
> flushes, by only flushing each pCPU once. It also allows multiple tlb
> flushes with only one vmexit.
> 
> hv_ipi allows sending inter-processor interrupts via vpindex, once again
> making it a prerequisite. Benefits are pretty much as with tlbflush.
> 
> hv_evmcs is a VM control structure in L1 guest memory, allowing an L1 guest
> to modify L2 VMCS and entering L2 without having the L0 host perform an
> expensive VMCS update on trapping the nested vmenter.
> 
> Signed-off-by: Stefan Reiter <s.reiter at proxmox.com>
> ---
> 
> v1 -> v2:
>     * Added commit description
>     * Fixed formatting (sorry)
>     * Changed hv_ipi and hv_evmcs to QEMU version 3.1 only
> 
> The last one was my mistake, I forgot a step in my testing setup for v1.
> ipi and evmcs are only supported in QEMU 3.1+, although kernel support
> is still present since 4.18/4.17. Since only 3.0 is rolled out, this is
> now preparation for the future I guess.
> 
> Live migration, both up and down versions works fine in my testing,
> as long as the target systems kernel is version 4.18+. As far as I'm
> aware, CPU feature flags like all of the hv_* ones are only checked on
> guest bootup. Our code already strips them from the target command line,
> so QEMU is working fine, and KVM already supports the hypercalls.
> 
> Migration to systems running older kernels will probably fail.
> 
> The microsoft Hyper-V spec is a good source for deeper information:
> https://github.com/MicrosoftDocs/Virtualization-Documentation/raw/live/tlfs/Hypervisor%20Top%20Level%20Functional%20Specification%20v5.0C.pdf
> 

applied, much thanks for your patience, the extensive test and the
now thorough information/commit message!
I still move the tlbflush into the 3.1 machine feature guard, just
to be on the safe side.