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Re: [PATCH 1/2] x86/time: use fake read_tsc()
- To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 1 Mar 2022 15:43:51 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 01 Mar 2022 14:44:00 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 01.03.2022 15:39, Andrew Cooper wrote:
> On 01/03/2022 14:14, Jan Beulich wrote:
>> On 01.03.2022 14:07, Andrew Cooper wrote:
>>> On 01/03/2022 11:05, Jan Beulich wrote:
>>> That said... what's wrong a plain NULL? I can't see any need for a
>>> magic constant here.
>> Are you fancying an XSA for a call through NULL in PV guest context?
>
> Why do you think that a risk? Only non-NULL function pointers are
> followed, and altcall resolves safely if the pointer is still NULL.
>
> And on that subject, don't we not hit the altcall's BUG_ON() for
> exceeding disp32?
There's no altcall involved here. As said in earlier contexts, altcall
patching comes to early to cover plt_tsc use. Hence the only concern
is a non-altacll-ed use of the pointer.
Jan
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