linux-kernel - Re: [PATCH RFC 00/15] KVM: Dirty ring interface

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Date:   Thu, 5 Dec 2019 20:59:33 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 00/15] KVM: Dirty ring interface

On 05/12/19 20:30, Peter Xu wrote:
>> Try enabling kvmmmu tracepoints too, it will tell
>> you more of the path that was taken while processing the EPT violation.
>
> These new tracepoints are extremely useful (which I didn't notice
> before).

Yes, they are!

> So here's the final culprit...
> 
> void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
> {
>         ...
> 	spin_lock(&kvm->mmu_lock);
> 	/* FIXME: we should use a single AND operation, but there is no
> 	 * applicable atomic API.
> 	 */
> 	while (mask) {
> 		clear_bit_le(offset + __ffs(mask), memslot->dirty_bitmap);
> 		mask &= mask - 1;
> 	}
> 
> 	kvm_arch_mmu_enable_log_dirty_pt_masked(kvm, memslot, offset, mask);
> 	spin_unlock(&kvm->mmu_lock);
> }
> 
> The mask is cleared before reaching
> kvm_arch_mmu_enable_log_dirty_pt_masked()..

I'm not sure why that results in two vmexits?  (clearing before
kvm_arch_mmu_enable_log_dirty_pt_masked is also what
KVM_{GET,CLEAR}_DIRTY_LOG does).

> The funny thing is that I did have a few more patches to even skip
> allocate the dirty_bitmap when dirty ring is enabled (hence in that
> tree I removed this while loop too, so that has no such problem).
> However I dropped those patches when I posted the RFC because I don't
> think it's mature, and the selftest didn't complain about that
> either..  Though, I do plan to redo that in v2 if you don't disagree.
> The major question would be whether the dirty_bitmap could still be
> for any use if dirty ring is enabled.

Userspace may want a dirty bitmap in addition to a list (for example:
list for migration, bitmap for framebuffer update), but it can also do a
pass over the dirty rings in order to update an internal bitmap.

So I think it make sense to make it either one or the other.

Paolo