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Description of problem:
With the currently recommended CPU flags:
-cpu ...,hv_time,hv_relaxed,hv_vapic,hv_spinlocks=0x1fff
modern Windows guests use the Hyper-V TSC page as a timestamp source and the RTC chip (MC146818) for timer interrupts.
The TSC page is fine but the RTC chip doesn't have a virt-friendly access protocol. One timer interrupt usually costs four extra VM exits as Windows executes this sequence twice:
outportb(0x70, 0x0C);
inportb(0x71);
the first inportb returns the contents of register C (usually 0xC0 - periodic timer interrupt) and clears it. Second inportb returns 0.
Version-Release number of selected component (if applicable):
kernel-3.10.0-686.el7 or 4.10.11 upstream
qemu-kvm-rhev-2.9.0-12.el7 or 2.9 upstream
Windows Server 2016 (en_windows_server_2016_x64_dvd_9327751.iso)
How reproducible:
100%
Steps to Reproduce:
1. Install Windows Server 2016 with -cpu ...,hv_time,hv_relaxed,hv_vapic,hv_spinlocks=0x1fff
2. Wait for it to become idle
3. Watch VM exit statistics using e.g. kvm_stat
Actual results:
~450 VM exits a second
out of which ~290 are I/O exits
Expected results:
Fewer VM exits :)
Vadim pointed me to the WAET ACPI table, documented by Microsoft in:
https://msdn.microsoft.com/en-us/windows/hardware/gg487524.aspx
I have tried providing this table to Windows guests using:
-acpitable sig=WAET,data=waet_data
where waet_data is four bytes long and contains 0x01, 0x00, 0x00, 0x00.
Windows Vista and Win7 really picks it up but fails to boot because the MC146818 implementation in QEMU is not compatible with the "RTC good" flag.
Newer Windows completely ignores the "RTC good" flag. This has been verified by disassembling hal.dll.
Even new Windows seem to pick up the other documented flag "ACPI PM timer good". Windows uses it only for HW detection and not for actual PM timer reads though. Even if it affected PM timer reads, it is of dubious value as Windows correctly prefers the TSC MSR / TSC page if available and doesn't use the PM timer except during boot.
We should consider adding hv_synic,hv_stimer to the recommended cpu flags for Windows guests. Using the synthetic timer instead of RTC chip significantly reduces the number of VM exits.
RTC timer: ~450 VM exits/second
Synthetic timer: ~130 exits/second
Backport Hyper-V TSC page enlightenment support
(https://bugzilla.redhat.com/show_bug.cgi?id=1504524)
should significantly improve (Ke)QueryPerformanceCount performance.
There is no any other option to make it working better at the moment.
Closing this bug as CURRENTRELEASE.
Best regards,
Vadim.
Comment 31Red Hat Bugzilla
2023-09-14 04:00:02 UTC
The needinfo request[s] on this closed bug have been removed as they have been unresolved for 1000 days