Re: RFC: KVM: x86/mmu: Eager Page Splitting

From: Paolo Bonzini <pbonzini@redhat.com>
To: David Matlack <dmatlack@google.com>
Cc: kvm list <kvm@vger.kernel.org>, Ben Gardon <bgardon@google.com>,
	Junaid Shahid <junaids@google.com>,
	Sean Christopherson <seanjc@google.com>,
	Oliver Upton <oupton@google.com>,
	Harish Barathvajasankar <hbarath@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Peter Xu <peterx@redhat.com>,
	Peter Shier <pshier@google.com>
Subject: Re: RFC: KVM: x86/mmu: Eager Page Splitting
Date: Mon, 8 Nov 2021 22:37:50 +0100	[thread overview]
Message-ID: <ba65851f-4f03-e5e2-ac88-139d9b48d44c@redhat.com> (raw)
In-Reply-To: <YYmBMGvU/kthFiM9@google.com>

On 11/8/21 20:57, David Matlack wrote:
>> I'm not super
>> interested in adding eager page splitting for the older methods (clear
>> on KVM_GET_DIRTY_LOG, and manual-clear without initially-all-set), but
>> it should be useful for the ring buffer method and that *should* share
>> most of the code with the older methods.
> 
> Using Eager Page Splitting with the ring buffer method would require
> splitting the entire memslot when dirty logging is enabled for that
> memslot right? Are you saying we should do that?

Yeah, that's why I said it should share code with clear-on-get-dirty.

For initially-all-set, where it's possible to do it and even easy-ish, I 
would like to avoid paying the cost of splitting entirely upfront, when 
enabling dirty page tracking.  But you can already post an RFC that just 
splits always when dirty page tracking is enabled, so that I have a bit 
more of an idea of the new code, and of what it would entail to smear 
the cost over the calls to KVM_CLEAR_DIRTY_LOG.

Thanks,

Paolo