Re: [Xen-devel] [PATCH v5 4/9] xen/x86: populate PVHv2 Dom0 physical memory map

[Tim Deegan <tim@xxxxxxx>]

At 09:40 -0700 on 27 Jan (1485510008), Jan Beulich wrote:
> >>> On 27.01.17 at 14:20, <tim@xxxxxxx> wrote:
> > At 12:51 +0000 on 27 Jan (1485521470), Andrew Cooper wrote:
> >> On 27/01/17 11:14, Tim Deegan wrote:
> >> > But looking at it now, I'm not convinced of exactly how.  The magic
> >> > bitmap in the TSS is at [I/O Map Base Address] - 32, and the I/O map
> >> > base address itself lives at offset 100.  A zero'd TSS should mean an
> >> > I/O map at 0, and an interrupt redirection bitmap at -32, which would
> >> > plausibly work if the TSS were 256 bytes (matching the limit set in
> >> > Xen).  Perhaps it's only working because the 128 bytes following the
> >> > TSS in hvmloader happen to be zeros too?
> >> 
> >> With an IO_base_map of 0, the software interrupt bitmap will end up
> >> being ahead of the TSS, not after it.
> > 
> > I should have thought that the segmented address calculation would
> > wrap and leave us at TSS + 224.
> 
> I don't think wrapping takes the limit value into account.

Quite right, I'm talking nonsense.

> - vmx_set_segment_register() will need to set a correct limit

Yep.

> - vmx_set_segment_register() should initialize the TSS every
>   time (including setting the I/O bitmap address to no lower
>   than 32)

Probably to no lower than 136, to avoid having the bits of that field
itself appearing in either the IO or interrupt bitmap.

> - hvmloader's init_vm86_tss() will need to allocate 160 bytes
>   rather than 128 (and we should expose this number, so that
>   Roger can also use it)
> 
> Perhaps we should even introduce a hypercall for hvmloader
> to query the needed value, rather than exposing a hardcoded
> number?

I think Andrew's suggestion of just using a whole page is a good
one.  The TSS is a 32-bit one, after all, and doesn't need to live in
BIOS space.

Cheers,

Tim.

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