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Re: [Xen-devel] [PATCH][HVM] Fix guest hang producing "PIT: bad access"messages

  • To: Christoph Egger <Christoph.Egger@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
  • Date: Fri, 11 Jan 2008 11:52:45 +0000
  • Delivery-date: Fri, 11 Jan 2008 03:53:29 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AchUSHqFuVoA+MA7EdymsQAX8io7RQ==
  • Thread-topic: [Xen-devel] [PATCH][HVM] Fix guest hang producing "PIT: bad access"messages

Okay, but the patch is buggy (accesses count*size bytes of memory, but
actually only one byte-sized port will be accessed) and actually this is
indicative of a wider issue in other Xen-emulated devices. We need a more
cooked interface to PIO handlers. I'll take a look.

 -- Keir

On 11/1/08 11:08, "Christoph Egger" <Christoph.Egger@xxxxxxx> wrote:

> Hi
> When the HVM guest atomically reads two bytes from the PIT via  "rep insb"
> and the two bytes crosses pages, then in xen/arch/x86/hvm/i8254.c,
> handle_pit_io(), p->data_is_ptr is 1 which leads to an unhandled read.
> Since reading the first byte does not change the guest rip, an endless loop
> of VMEXIT_IOIO happens. This leads to an endless stream of "PIT: bad access"
> xen messages => Guest hangs.
> This has been accurately reproduced on NetBSD/amd64 as HVM guest
> and this also been seen on 64bit Windows guests.
> The attached patch handles the page-crossing access in the PIT emulation.
> Signed-off-by: Christoph Egger <Christoph.Egger@xxxxxxx>

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