Re: [Xen-devel] Fetching instructions after page-fault, near page boundary?

[Anthony Liguori <aliguori@xxxxxxxxxx>]

I would think you would not only have to worry about crossing page 
boundaries, but also crossing a segment descriptor limit.  These days, 
segmentation is used for some security purposes (to emulate a NX bit for 
instance).
Regards,

Anthony Liguori

Petersson, Mats wrote:
> If we get a page-fault due to a MMIO access to a virtual MMIO device
> (such as VGA screen in HVM), we shouldn't need to worry about crossing
> the page-boundary at the end of the instruction, right? Let's say the
> instruction is a 7-byte instruction like this:
>
> xxxx1FFD: 11 22 33 <page boundary to page xxxx2000> 44 55 66 77
>
> If the page xxxx2000 isn't present when the instruction is started, then
> we'd FIRST get a page-fault for this address, so either we fail the
> instruction (if xxxx2000 page isn't actually possible to be fixed up),
> or we get the page fixed up and therefore the second time, when we get
> to the page-fault handler looking at the address the instruction is
> accessing [doing the MMIO part], the second page is present [assuming we
> haven't got any sneaky code going round modifying the page-tables for
> this guest domain - which I don't think is a VALID thing to expect, is
> it?]
>
> Next case is where we have a short instruction before an empty(unused
> page), say a three-byte instruction (RR is another instructon, such as a
> return instruction). 
>
> xxx1FFC: 11 22 33 RR <page boundary to xxxx2000> [not readable since
> it's not present]. 
>
> My design idea for the merged x86_emulate.c in QEMU is to read
> instruction bytes blind (i.e. not knowing the actual instruction length)
> by the this method:
> Try to read 15 bytes (MAX_INST_LEN), and if the instruction bytes happen
> to cross a page-boundary, and the second page is not readable, I'll just
> cut the number of bytes short, assuming that the valid instruction is
> shorter than 15 bytes. 
>
> Does anyone see a problem with this method?
>
> [By the way, this makes an improvement over the current setup, which
> fails if we try to read a page that isn't readable - which at least the
> SVM model does try sometimes]. 
>
> --
> Mats
>
>
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