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

[Peter Xu]

On Fri, Feb 07, 2020 at 11:45:32AM -0800, Sean Christopherson wrote:
> +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.
> 

I proposed the 4th solution here:

https://lore.kernel.org/kvm/20200207223520.735523-1-peterx@xxxxxxxxxx/

I'm not sure whether that's acceptable, but if it can, then we can
drop the kvm_arch_dirty_log_tlb_flush() hook, or even move on to
per-slot tlb flushing.

Thanks,

-- 
Peter Xu