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

From: Vitaly Kuznetsov
Date: Mon Feb 17 2020 - 10:35:13 EST

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> +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?

(Sorry for the delayed reply, was traveling last week)

When KVM runs as an L1 on top of Hyper-V it uses eVMCS by default and
eVMCSv1 doesn't support PML. I've also just checked Hyper-V 2019 and it
hides SECONDARY_EXEC_ENABLE_PML from guests (this was expected).

> 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'd vote for (2): while this will effectively be kvm_flush_remote_tlbs()
for now, we may think of something smarter in the future (e.g. PV
interface for KVM-on-KVM).

> 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.