Re: [Xen-devel] [V10 PATCH 09/23] PVH xen: introduce pvh_set_vcpu_info() and vmx_pvh_set_vcpu_info()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.08.13 at 03:53, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:
> On Mon, 5 Aug 2013 18:34:36 -0700
> Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:
> 
>> On Mon, 05 Aug 2013 12:08:50 +0100
>> "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>> 
>> > >>> On 26.07.13 at 02:58, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>
>> > >>> wrote:
>> > > On Thu, 25 Jul 2013 14:47:55 +0100
>> > > Tim Deegan <tim@xxxxxxx> wrote:
> ....... > > 
>> > >> At 18:59 -0700 on 23 Jul (1374605957), Mukesh Rathor wrote:
>> > > That was an option discussed with Jan, walking and reading the GDT
>> > > entries from the gdtaddr the guest provided to load the hidden
>> > > parts. But, I agree with him, that for the initial cpu boot we can
>> > > restrict the ABI to say: 0 base addr, ~0 limit, and "read/write,
>> > > accessed" default attributes for the hidden part (64bit guest). 
>> > 
>> > That must be a misunderstanding then (also see my other reply) - I
>> > always meant to require that you either properly load the hidden
>> > register portions from the descriptor tables, or at least verify
>> > that the descriptor table entries referenced match the defaults you
>> > enforce.
>> 
>> Ok, I thought you just wanted to be documented, If I'm gonna write
>> the code to verify, i might as well just write the hidden porions, an
>> option I'd proposed. That way there are no constraints. I'm currently
>> working on just doing that, and will be in the next version of the
>> patch.
> 
> Ok, I've mostly got code to set the hidden fields, but the more I think
> about it, the more I feel that the right/better thing to do is to
> just not set any selectors at all, instead default them in the VMCS.
> Meaning, we set CS=0x10, DS/SS = 0x18  in vmcs create code. Then we
> document that the guest needs to set a boot GDT as:
> 
>  0000000000000000 00cf9b000000ffff
>  00af9b000000ffff 00cf93000000ffff
> 
> to bootstrap. PV does this anyways, and since we are dealing with PVH
> here, which is still modified guest, it seems fairly plausible. The
> guest just needs to send down the GDT addr and size for the boot vcpu
> then as part of this ABI. Recall this is just for the intial boot of the
> vcpu, once its up, it manages its own GDT.
> 
> What do you guys think? please lmk.

I don't mind doing it that way, as long as (as said before) the
code properly verifies these constraints at least in debug mode.

However, if you want to go with a "minimal" concept, then let's
please require a truly minimal GDT: For a 64-bit guest that'd be
CS = 0x0008, all other selector registers zero. All you need to
validate then is a single descriptor, and the guest could even
choose to point the initial GDT at the "normal" GDT's 64-bit code
descriptor minus 8.

Jan


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