Re: [Xen-devel] debian stretch dom0 + xen 4.9 fails to boot

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.06.17 at 14:42, <Paul.Durrant@xxxxxxxxxx> wrote:
> For those following this...
> 
> By poking characters at the screen and bisecting where they stopped, I have 
> narrowed the problem to the code in edd.S. I can successfully boot by setting 
> opt_edd=off on the Xen cmd line and I can also boot with the following patch 
> applied:
> 
> diff --git a/xen/arch/x86/boot/edd.S b/xen/arch/x86/boot/edd.S
> index 73371f98b5..5409f1d9a1 100644
> --- a/xen/arch/x86/boot/edd.S
> +++ b/xen/arch/x86/boot/edd.S
> @@ -148,5 +148,6 @@ GLOBAL(boot_mbr_signature_nr)
>          .byte   0
>  GLOBAL(boot_mbr_signature)
>          .fill   EDD_MBR_SIG_MAX*8,1,0
> +       .align  4096
>  GLOBAL(boot_edd_info)
> -        .fill   512,1,0                         # big enough for a disc 
> sector
> +        .fill   4096,1,0                         # big enough for a disc 
> sector
> 
> (based on a hunch that the BIOS defaults to a 4K sector for my NVMe drive)
> 
> I need to investigate some more but I do wonder whether the EDD info should 
> be read first to determine the appropriate size of memory buffer to use when 
> issuing the read of the MBR. Hardcoding a 4k reservation seems like the wrong 
> thing to do, even if it is sufficient for this BIOS.

boot_edd_info is being used for two things - reading the MBR of
each disk and storing data retrieved from INT 13 Fn 41 and 48.
The latter occupies 492 bytes (6 times 8+74). Which would make
me guess the system has a 4k disk, and the BIOS doesn't abstract
away this characteristic when handling INT 13 Fn 02 (which is
supposed to only act in multiples of 512-byte sectors, as opposed
to Fn 42).

The alternative of Fn 48 overflowing its buffer would seem less
likely, especially with the buffer holding a size on input.

Do you, btw, really need both the size and alignment increases?

Jan


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