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

["Christoph Egger" <Christoph.Egger@xxxxxxx>]

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>


-- 
AMD Saxony, Dresden, Germany
Operating System Research Center

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Attachment: xen_pit.diff
Description: Text document

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