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From: Jan Beulich <jbeulich@suse.com>
To: Kevin Negy <kevinnegy@gmail.com>
Cc: xen-devel@lists.xenproject.org
Subject: Re: How does shadow page table work during migration?
Date: Fri, 19 Feb 2021 17:36:25 +0100	[thread overview]
Message-ID: <4d3a6f57-31e3-3709-4ed1-a39b5fe55347@suse.com> (raw)
In-Reply-To: <CACZWC-qK_biKgyi+ZiXnsHRscAbK9pz=kncdBA25QYWY129HCQ@mail.gmail.com>

On 19.02.2021 17:10, Kevin Negy wrote:
> I'm trying to understand how the shadow page table works in Xen,
> specifically during live migration. My understanding is that after shadow
> paging is enabled (sh_enable_log_dirty() in
> xen/arch/x86/mm/shadow/common.c), a shadow page table is created, which is
> a complete copy of the current guest page table. Then the CR3 register is
> switched to use this shadow page table as the active table while the guest
> page table is stored elsewhere. The guest page table itself (and not the
> individual entries in the page table) is marked as read only so that any
> guest memory access that requires the page table will result in a page
> fault. These page faults happen and are trapped to the Xen hypervisor. Xen
> will then update the shadow page table to match what the guest sees on its
> page tables.
> Is this understanding correct?

Partly. For HVM, shadow mode (if so used) would be active already. For
PV, page tables would be read-only already. Log-dirty mode isn't after
page table modifications alone, but to notice _any_ page that gets
written to.

> If so, here is where I get confused. During the migration pre-copy phase,
> each pre-copy iteration reads the dirty bitmap (paging_log_dirty_op() in
> xen/arch/x86/mm/paging.c) and cleans it. This process seems to destroy all
> the shadow page tables of the domain with the call to shadow_blow_tables()
> in sh_clean_dirty_bitmap().
> How is the dirty bitmap related to shadow page tables?

Shadow page tables are the mechanism to populate the dirty bitmap.

> Why destroy the
> entire shadow page table if it is the only legitimate page table in CR3 for
> the domain?

Page tables will get re-populated again as the guest touches memory.
Blowing the tables is not the same as turning off shadow mode.


  reply	other threads:[~2021-02-19 16:36 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-19 16:10 How does shadow page table work during migration? Kevin Negy
2021-02-19 16:36 ` Jan Beulich [this message]
2021-02-19 20:17 ` Andrew Cooper
2021-02-22 18:52   ` Kevin Negy
2021-02-23  0:23     ` Andrew Cooper

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