Re: [PATCH] x86: kvm: hyper-v: deal with buggy TLB flush requests from WS2012

From: Vitaly Kuznetsov
Date: Thu Apr 18 2019 - 03:14:09 EST

Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> writes:

> It was reported that with some special Multi Processor Group configuration,
> e.g:
> bcdedit.exe /set groupsize 1
> bcdedit.exe /set maxgroup on
> bcdedit.exe /set groupaware on
> for a 16-vCPU guest WS2012 shows BSOD on boot when PV TLB flush mechanism
> is in use.
> Tracing kvm_hv_flush_tlb immediately reveals the issue:
> kvm_hv_flush_tlb: processor_mask 0x0 address_space 0x0 flags 0x2
> The only flag set in this request is HV_FLUSH_ALL_VIRTUAL_ADDRESS_SPACES,
> however, processor_mask is 0x0 and no HV_FLUSH_ALL_PROCESSORS is specified.
> We don't flush anything and apparently it's not what Windows expects.
> TLFS doesn't say anything about such requests and newer Windows versions
> seem to be unaffected. This all feels like a WS2012 bug, which is, however,
> easy to workaround in KVM: let's flush everything when we see an empty
> flush request, over-flushing doesn't hurt.

Ping) I understand the lack of interest towards this ugly hack but I
don't see a better option ...