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

  • To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
  • From: "Kip Macy" <kip.macy@xxxxxxxxx>
  • Date: Wed, 20 Feb 2008 13:56:48 -0800
  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 20 Feb 2008 13:57:19 -0800
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On Wed, Feb 20, 2008 at 12:10 AM, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:
> 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.

Ok. That isn't what I'm seeing on 3.1 and 3.2, I'll instrument xen and
try to track it down.

Also, I removed the use of recursive page tables so that I could use
writeable page tables - not realizing that wr pts were a dud. I've
disabled writeable page tables and have tried re-enabling the use of
recursive page tables. It appears that that doesn't work at all any
more - is that correct?


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