Re: [PATCH v2] x86: kvm: hyper-v: deal with buggy TLB flush requests from WS2012

[Sean Christopherson]

On Tue, Apr 23, 2019 at 03:15:37PM +0200, Vitaly Kuznetsov wrote:
> 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.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---

Reviewed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>