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

[Sean Christopherson]

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@xxxxxxxxxx>
> ---
>  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.
		 */


> +		 * 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
>