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

[Stefan Reiter]

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

 PVE/QemuServer.pm | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
index 341e0b0..aff0915 100644
--- a/PVE/QemuServer.pm
+++ b/PVE/QemuServer.pm
@@ -7170,6 +7170,15 @@ sub add_hyperv_enlightenments {
 	    push @$cpuFlags , 'hv_synic';
 	    push @$cpuFlags , 'hv_stimer';
 	}
+
+	if (qemu_machine_feature_enabled ($machine_type, $kvmver, 3, 0)) {
+	    push @$cpuFlags , 'hv_tlbflush';
+	}
+
+	if (qemu_machine_feature_enabled ($machine_type, $kvmver, 3, 1)) {
+	    push @$cpuFlags , 'hv_ipi';
+	    push @$cpuFlags , 'hv_evmcs';
+	}
     }
 }
 
-- 
2.20.1