Subject: Re: [PATCH v5 15/19] KVM: Provide common implementation for generic dirty log functions

From: Sean Christopherson <[email-masked]>
Date: Fri, 7 Feb 2020 11:45:32 -0800

+Vitaly for HyperV

On Thu, Feb 06, 2020 at 04:41:06PM -0500, Peter Xu wrote:
> On Thu, Feb 06, 2020 at 01:21:20PM -0800, Sean Christopherson wrote:
> > On Thu, Feb 06, 2020 at 03:02:00PM -0500, Peter Xu wrote:
> > > But that matters to this patch because if MIPS can use
> > > kvm_flush_remote_tlbs(), then we probably don't need this
> > > arch-specific hook any more and we can directly call
> > > kvm_flush_remote_tlbs() after sync dirty log when flush==true.
> > 
> > Ya, the asid_flush_mask in kvm_vz_flush_shadow_all() is the only thing
> > that prevents calling kvm_flush_remote_tlbs() directly, but I have no
> > clue as to the important of that code.
> 
> As said above I think the x86 lockdep is really not necessary, then
> considering MIPS could be the only one that will use the new hook
> introduced in this patch...  Shall we figure that out first?

So I prepped a follow-up patch to make kvm_arch_dirty_log_tlb_flush() a
MIPS-only hook and use kvm_flush_remote_tlbs() directly for arm and x86,
but then I realized x86 *has* a hook to do a precise remote TLB flush.
There's even an existing kvm_flush_remote_tlbs_with_address() call on a
memslot, i.e. this exact scenario.  So arguably, x86 should be using the
more precise flush and should keep kvm_arch_dirty_log_tlb_flush().

But, the hook is only used when KVM is running as an L1 on top of HyperV,
and I assume dirty logging isn't used much, if at all, for L1 KVM on
HyperV?

I see three options:

  1. Make kvm_arch_dirty_log_tlb_flush() MIPS-only and call
     kvm_flush_remote_tlbs() directly for arm and x86.  Add comments to
     explain when an arch should implement kvm_arch_dirty_log_tlb_flush().

  2. Change x86 to use kvm_flush_remote_tlbs_with_address() when flushing
     a memslot after the dirty log is grabbed by userspace.

  3. Keep the resulting code as is, but add a comment in x86's
     kvm_arch_dirty_log_tlb_flush() to explain why it uses
     kvm_flush_remote_tlbs() instead of the with_address() variant.

I strongly prefer to (2) or (3), but I'll defer to Vitaly as to which of
those is preferable.

I don't like (1) because (a) it requires more lines code (well comments),
to explain why kvm_flush_remote_tlbs() is the default, and (b) it would
require even more comments, which would be x86-specific in generic KVM,
to explain why x86 doesn't use its with_address() flush, or we'd lost that
info altogether.

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