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

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Message-ID: <87sguf23b8.fsf@vitty.brq.redhat.com>
Date:   Thu, 18 Apr 2019 09:14:03 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: kvm: hyper-v: deal with buggy TLB flush requests from WS2012

Vitaly Kuznetsov <vkuznets@...hat.com> 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 ...

-- 
Vitaly