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

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Message-ID: <20190423135934.GA10720@linux.intel.com>
Date:   Tue, 23 Apr 2019 06:59:34 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH v2] x86: kvm: hyper-v: deal with buggy TLB flush requests
 from WS2012

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@...hat.com>
> ---

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