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Re: [PATCH 1/4] x86/P2M: write_p2m_entry() is HVM-only anyway


  • To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 23 Apr 2024 20:29:40 +0100
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  • Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>
  • Delivery-date: Tue, 23 Apr 2024 19:29:54 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 23/04/2024 3:31 pm, Jan Beulich wrote:
> The latest as of e2b2ff677958 ("x86/P2M: split out init/teardown
> functions") the function is obviously unreachable for PV guests.

This doesn't parse.  Do you mean "Since e2b2ff677958 ..." ?

>  Hence
> the paging_mode_enabled(d) check is pointless.
>
> Further host mode of a vCPU is always set, by virtue of
> paging_vcpu_init() being part of vCPU creation. Hence the
> paging_get_hostmode() check is pointless.
>
> With that the v local variable is unnecessary too. Drop the "if()"
> conditional and its corresponding "else".
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> I have to confess that this if() has been puzzling me before.

Puzzling yes, but it can't blindly be dropped.

This is the "did the toolstack initiate this update" check.  i.e. I
think it's "bypass the normal side effects of making this update".

I suspect it exists because of improper abstraction between the guest
physmap and the shadow pagetables as-were - which were/are tighly
coupled to vCPUs even for aspects where they shouldn't have been.

For better or worse, the toolstack can add_to_physmap() before it
creates vCPUs, and it will take this path you're trying to delete. 
There may be other cases too; I could see foreign mapping ending up
ticking this too.

Whether we ought to permit a toolstack to do this is a different
question, but seeing as we explicitly intend (eventually for AMX) have a
set_policy call between domain_create() and vcpu_create(), I don't think
we can reasably restrict other hypercalls too in this period.

~Andrew



 


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