linux-kernel - Re: [PATCH 15/15] KVM: x86/mmu: Promote pages in-place when disabling dirty logging

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Date:   Wed, 1 Dec 2021 09:59:23 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Peter Shier <pshier@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Kai Huang <kai.huang@...el.com>,
        Keqian Zhu <zhukeqian1@...wei.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 15/15] KVM: x86/mmu: Promote pages in-place when
 disabling dirty logging

On Tue, Nov 30, 2021 at 04:01:50PM +0000, Sean Christopherson wrote:
> On Tue, Nov 30, 2021, Peter Xu wrote:
> > On Mon, Nov 29, 2021 at 10:31:14AM -0800, Ben Gardon wrote:
> > > 2. There could be a pointer to the page table in a vCPU's paging
> > > structure caches, which are similar to the TLB but cache partial
> > > translations. These are also cleared out on TLB flush.
> > 
> > Could you elaborate what's the structure cache that you mentioned?  I thought
> > the processor page walker will just use the data cache (L1-L3) as pgtable
> > caches, in which case IIUC the invalidation happens when we do WRITE_ONCE()
> > that'll invalidate all the rest data cache besides the writter core.  But I
> > could be completely missing something..
> 
> Ben is referring to the Intel SDM's use of the term "paging-structure caches"
> Intel CPUs, and I'm guessing other x86 CPUs, cache upper level entries, e.g. the
> L4 PTE for a given address, to avoid having to do data cache lookups, reserved
> bits checked, A/D assists, etc...   Like full VA=>PA TLB entries, these entries
> are associated with the PCID, VPID, EPT4A, etc...
> 
> The data caches are still used when reading PTEs that aren't cached in the TLB,
> the extra caching in the "TLB" is optimization on top.
> 
>   28.3.1 Information That May Be Cached
>   Section 4.10, “Caching Translation Information” in Intel® 64 and IA-32 Architectures
>   Software Developer’s Manual, Volume 3A identifies two kinds of translation-related
>   information that may be cached by a logical processor: translations, which are mappings
>   from linear page numbers to physical page frames, and paging-structure caches, which
>   map the upper bits of a linear page number to information from the paging-structure
>   entries used to translate linear addresses matching those upper bits.

Ah, I should have tried harder when reading the spec, where I just stopped at
4.10.2... :) They're also described in general section of 4.10.3 and also on
how TLB invalidations affect these caches in 4.10.4.

Thanks again to both!

-- 
Peter Xu