Re: [PATCH 06/11] KVM: MMU: let page fault handler be aware tracked page

From: Xiao Guangrong <guangrong.xiao@linux.intel.com>
To: Kai Huang <kai.huang@linux.intel.com>, pbonzini@redhat.com
Cc: gleb@kernel.org, mtosatti@redhat.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 06/11] KVM: MMU: let page fault handler be aware tracked page
Date: Tue, 15 Dec 2015 17:03:59 +0800	[thread overview]
Message-ID: <566FD77F.9050009@linux.intel.com> (raw)
In-Reply-To: <566FCB15.1000004@linux.intel.com>

On 12/15/2015 04:11 PM, Kai Huang wrote:
>
>
> On 12/01/2015 02:26 AM, Xiao Guangrong wrote:
>> The page fault caused by write access on the write tracked page can not
>> be fixed, it always need to be emulated. page_fault_handle_page_track()
>> is the fast path we introduce here to skip holding mmu-lock and shadow
>> page table walking
> Why can it be out side of mmu-lock? Is it OK that some other thread removes tracking of this page
> simultaneously? Shall we assuming the emulation code should handle this case?
>

What your mentioned is the worst case, if that happen the vcpu will spend
longer time to emulate the access rather then retry it. It is bad but it is
the rare contention. It is worth speeding up the common / most case.

> Even it works for write protection, is it OK for other purpose tracking (as your tracking framework
> can be extended for other purpose)?

We only need to make sure that no track event is lost, i.e, we can not
skip the case that the index is changed from 0 to 1.

If we see index is 0, the vcpu can hold the mmu-lock and go to slow path
anyway so no track event will be lost.

>>
>> However, if the page table is not present, it is worth making the page
>> table entry present and readonly to make the read access happy
> It's  fine for tracking write from guest. But what if I want to track every read from guest?
> Probably I am exaggerating :)
>

Then we do not go to the real page fault handler, just keep the shadow
page entry non-present.