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

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Date:   Thu, 18 Apr 2019 18:47:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org,
        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

On 18/04/19 16:17, Sean Christopherson wrote:
> On Wed, Mar 20, 2019 at 06:43:20PM +0100, 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>
>> ---
>>  arch/x86/kvm/hyperv.c | 12 +++++++++++-
>>  1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index 421899f6ad7b..5887f7d22ac6 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -1371,7 +1371,17 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *current_vcpu, u64 ingpa,
>>  
>>  		valid_bank_mask = BIT_ULL(0);
>>  		sparse_banks[0] = flush.processor_mask;
>> -		all_cpus = flush.flags & HV_FLUSH_ALL_PROCESSORS;
>> +
>> +		/*
>> +		 * WS2012 seems to be buggy, under certain conditions it is
>> +		 * possible to observe requests with processor_mask = 0x0 and
>> +		 * no HV_FLUSH_ALL_PROCESSORS flag set. It also seems that
> 
> "and no HV_FLUSH_ALL_PROCESSORS flag set" is awkward, and probably
> extraneous.  The whole comment is a probably a bit more verbose than it
> needs to be, e.g. most readers won't care how we came to the conclusion
> that 'processor_mask == 0', and those that care about the background will
> read the changelog anyways.
> 
> Maybe something like this:
> 
> 		/*
> 		 * Some Windows versions, e.g. WS2012, use processor_mask = 0
> 		 * in lieu of the dedicated flag to flush all processors.
> 		 */

Hmm, not really.  "In lieu" seems intentional.  "without" is more accurate.
My take:

                 * Work around possible WS2012 bug: it sends hypercalls
                 * with processor_mask = 0x0 and HV_FLUSH_ALL_PROCESSORS clear,
                 * while also expecting us to flush something and crashing if
                 * we don't. Let's treat processor_mask == 0 same as
                 * HV_FLUSH_ALL_PROCESSORS.
                 */

Paolo


> 
> 
>> +		 * Windows actually expects us to flush something and crashes
>> +		 * otherwise. Let's treat processor_mask == 0 same as
>> +		 * HV_FLUSH_ALL_PROCESSORS.
>> +		 */
>> +		all_cpus = (flush.flags & HV_FLUSH_ALL_PROCESSORS) ||
>> +			(flush.processor_mask == 0);
> 
> Nits:
> 
> Personal preference, but I like '!flush.processor_mask' in this case as it
> immediately conveys that we're handling the scenario where the guest didn't
> set a mask.  Then there wouldn't be a visual need for the second set of
> parentheses.
> 
> Aligning its indentation with the first first chunk of the statement would
> also be nice, but again, personal preference.  :-)
> 
>>  	} else {
>>  		if (unlikely(kvm_read_guest(kvm, ingpa, &flush_ex,
>>  					    sizeof(flush_ex))))
>> -- 
>> 2.20.1
>>