Re: [PATCH 0/15] KVM: optimize for MMIO handled

Avi Kivity
2011-06-09T07:39:16Z

On 06/07/2011 03:58 PM, Xiao Guangrong wrote:
> The idea of this patchset is from Avi:
> | We could cache the result of a miss in an spte by using a reserved bit, and
> | checking the page fault error code (or seeing if we get an ept violation or
> | ept misconfiguration), so if we get repeated mmio on a page, we don't need to
> | search the slot list/tree.
> | (https://lkml.org/lkml/2011/2/22/221)
>
> The aim of this patchset is to support fast mmio emulate, it reduce searching
> mmio gfn from memslots which is very expensive since we need to walk all slots
> for mmio gfn, and the other advantage is: we can reduce guest page table walking
> for soft mmu.
>
> Lockless walk shadow page table is introduced in this patchset, it is the light
> way to check the page fault is the real mmio page fault or something is running
> out of our mind.
>
> And, if shadow_notrap_nonpresent_pte is enabled(bypass_guest_pf=1), mmio page
> fault and normal page fault is mixed(the reserved is set for all page fault),
> it has little regression, if the box can generate lots of mmio access, for
> example, the network server, it can disable shadow_notrap_nonpresent_pte and
> enable mmio pf, after all, we can enable/disable mmio pf at the runtime.
>

Okay, this is pretty complicated. And things are already complicated.

First, I think we should consider dropping bypass_guest_pf completely,
just so we have less things to think about.

Second, I don't like two paths for accessing shadow page tables, it
makes the code much larger. I'm also not sure RCU is enough protection
- we can unlink a page in the middle of a hierarchy, and on i386 this
causes an invalid pointer to appear when we fetch the two halves. But I
guess, if the cpu can do it, so can we.

Maybe we can do something like

again:
fetch pointer to last level spte using RCU
if failed:
take lock
build spte hierarchy
drop lock
goto again
if sync:
if mmio:
do mmio
return
return
walk guest table
install spte
if mmio:
do mmio

(sync is always false for tdp)

--
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