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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 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.

(I haven't considered non-PAE case, TBH)

-boris

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