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Re: [PATCH] x86/domain: restrict context switch hooks for the idle domain
- To: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 2 Oct 2026 09:55:09 +0200
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- Cc: Grygorii Strashko <grygorii_strashko@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Fri, 02 Oct 2026 07:55:17 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02.10.2026 09:48, Roger Pau Monné wrote:
> On Fri, Oct 02, 2026 at 07:45:07AM +0200, Jan Beulich wrote:
>> On 01.10.2026 16:27, Roger Pau Monne wrote:
>>> --- a/xen/arch/x86/domain.c
>>> +++ b/xen/arch/x86/domain.c
>>> @@ -813,8 +813,6 @@ static bool emulation_flags_ok(const struct domain *d,
>>> uint32_t emflags)
>>> void __init arch_init_idle_domain(struct domain *d)
>>> {
>>> static const struct arch_csw idle_csw = {
>>> - .from = paravirt_ctxt_switch_from,
>>> - .to = paravirt_ctxt_switch_to,
>>> .tail = idle_loop,
>>> };
>>
>> Leaving NULL pointers around isn't a good thing, though. May I suggest to
>> at least poison the pointers then?
>
> I don't mind doing so, but I don't think we usually do that for other
> hook structures, ie: hvm_function_table for example doesn't poison
> unset hooks (and it possibly can't, because there are .hook != NULL
> checks).
>
> TBH I'm missing a benefit of the poisoning here. Using NULL or a
> poisoned value will both trigger a page-fault, and NULL has the
> benefit of callers easily checking whether the hook is set.
NULL in the context of a HVM vCPU will #PF. NULL in the context of a PV
one may not, depending on what the guest may have put there.
Jan
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