Re: [Xen-devel] [PATCH] docs: add PVH specification

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.09.14 at 13:00, <roger.pau@xxxxxxxxxx> wrote:
> El 17/09/14 a les 13.59, Jan Beulich ha escrit:
>>>>> On 16.09.14 at 17:53, <roger.pau@xxxxxxxxxx> wrote:
>>> +All the segment selectors (`cs`, `ds`, `ss`, `es`, `fs` and `gs`), the
>>> +`FS.base` and `GS.base` MSRs are zeroed out.
>> 
>> For the selector registers, specifying what the hidden portions hold
>> is a must I think, at the very least for %cs and %ss.
> 
> Done. I've added the following:
> 
> The `cs` segment selector is set by Xen with a base of 0x0 and a limit
> of 0xfffff. The attributes are set to 0x9b, which describes an
> executable and readable code segment only accessible by the most
> privileged level.

Considering that we're talking of 64-bit guests only at this point,
base and limit of %cs don't matter at all. What does matter and
is not mentioned above is that CS.L is set and CS.DB is clear. (And
if talking about the limit, 0xfffff is only the raw value - together
with CS.G it would end up being 0xffffffff.)

> The remaining segment selectors (`ds`, `ss`, `es`, `fs` and `gs`) are
> all set to the same values. Both the selector and the base is set to 0x0
> and the limit to 0xfffff. The attributes are set to 0x93, which implies
> a read and write data segment only accessible by the most privileged level.

Mostly the same here: Limit doesn't matter, and base matters only
for %fs and %gs. For other than %ss, most of the other attributes
(including privilege level) don't matter either.

That said, I don't mind spelling out what we do when there's no
foreseeable reason for us to ever change those. I.e. all I'd really
like to see changed is the attributes of CS to be fully specified,
and the confusion about the limit removed - if you want to keep the
information on base and limit, just say "flat at base zero" or some
such.

Jan


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