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Re: [Xen-devel] is there any way to avoid lazy updates to page attribute bits?

  • To: Kip Macy <kip.macy@xxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
  • Date: Wed, 20 Feb 2008 08:10:35 +0000
  • Delivery-date: Wed, 20 Feb 2008 00:10:01 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AchzmBG+UDIPet+LEdyG0wAWy6hiGQ==
  • Thread-topic: [Xen-devel] is there any way to avoid lazy updates to page attribute bits?

On 20/2/08 02:34, "Kip Macy" <kip.macy@xxxxxxxxx> wrote:

> I'm finding that regardless of which update method I use - writeable
> page tables, HYPERVISOR_update_va_mapping, or HYPERVISOR_mmu_update
> that xen will frequently only update the address bits in the page
> table at the time of the hypercall - presumably setting the attribute
> bits some time later when the mapping is accessed. This is fairly
> problematic for me in that freebsd relies on the page attribute bits
> in the page table accurately reflecting the attributes that were set
> for a mapping. Is it now necessary for PV guests to maintain page
> attribute data outside of the page tables?

Attributes are always updated synchronously at the same time as the address
bits. Xen may modify the attributes a bit (e.g., fiddling with PAGE_GLOBAL
and PAGE_USER for 64-bit guests) but this is done deterministically and
synchronously; never lazily.

 -- Keir

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