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

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Message-ID: <YaZK7lxaBMGfYIdz@google.com>
Date:   Tue, 30 Nov 2021 16:01:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Xu <peterx@...hat.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, 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.