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Re: [Xen-devel] [PATCH v4 2/4] x86/compat: Test both PV and PVH guests for compat mode
On 09/02/2015 09:13 AM, Jan Beulich wrote:
On 02.09.15 at 14:31, <boris.ostrovsky@xxxxxxxxxx> wrote:
On 09/02/2015 04:08 AM, Jan Beulich wrote:
Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 09/02/15 2:55 AM >>>
On 08/27/2015 12:01 PM, Jan Beulich wrote:
On 13.08.15 at 20:12, <boris.ostrovsky@xxxxxxxxxx> wrote:
@@ -777,7 +777,7 @@ int arch_set_info_guest(
/* The context is a compat-mode one if the target domain is
compat-mode;
* we expect the tools to DTRT even in compat-mode callers. */
- compat = is_pv_32bit_domain(d);
+ compat = is_pv_32bit_domain(d) || is_pvh_32bit_domain(d);
I continue to think that this should include a v->domain ==
current->domain check (to match behavior for HVM guests
from the tool stack perspective). Having looked at patch 4, I also
can't see how the tool stack is being made expect a non-native
guest context record in the 32-bit PVH case (i.e. I'd appreciate
if you could point out where that hides).
For vcpu 0 current->domain is dom0 so I am not sure how this check would
work.
For a 32-bit PVH guest the toolstack will place data into
vcpu_guest_context_x86_32_t (in vcpu_x86_32()) and so the hypervisor,
knowing that the guest is a compat one (based on the test above), will
access appropriate fields.
This is not how HVM guests are started --- "classic" PVH behaves very
much like a PV guest, unlike what we are doing with no-dm PVH.
And I believe this to be wrong, and potentially getting in the way of the
no-dm
work - Roger?
As to the reference to vcpu_x86_32() - by its name alone it is already clear
that this is after the determination of what bit width a guest to deal with, and
looking at xc_dom_32_pae I still can't see why PVH guests would (intentionally
and legitimately) be treated like PV rather than HVM ones (not to speak of the
fact that I don't think 32-bit PVH is in any way limited to PAE).
The purpose of this series is to get 32-bit guests to parity with
classic PVH with minimal changes and then move on to no-dm. Not getting
in the way of no-dm is obviously important but making classic behave
like no-dm (which is, to certain extent, is what you are suggesting) is
out of scope.
Well, okay. But are you saying then that 64-bit PVH also just
_happens_ to be treated like PV in the tool stack?
Yes. They both are processed by tollstack's elf parser (and the size is
determined by xc_dom_guest_type()).
IOW I'm still missing
the explicit tool stack adjustment that makes it use a guest-bit-width
context for PVH guests.
(I haven't considered non-PAE case, TBH)
But I'm afraid you need to.
Actually, at least for Linux, CONFIG_XEN 'depends on X86_64 || (X86_32
&& X86_PAE)'. And I don't know whether there are any other OSs (i.e.
FreeBSD) that are going to ever support 32-bit PVH-classic (Roger might
correct me on that).
-boris
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