Re: [Xen-devel] [PATCH RFC 4/4] xen: use per-vcpu TSS and stacks for pv domains

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

(sorry for the top-post. I'm on my phone) 

I can see you are using ltr, but I don't see anywhere where where you are 
changing the content on the TSS, or the top-of-stack content.

It is very complicated to safely switch IST stacks when you might be taking 
interrupts. 

~Andrew 
________________________________________
From: Juergen Gross [jgross@xxxxxxxx]
Sent: 09 January 2018 17:40
To: Andrew Cooper; xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: Ian Jackson; konrad.wilk@xxxxxxxxxx; jbeulich@xxxxxxxx
Subject: Re: [PATCH RFC 4/4] xen: use per-vcpu TSS and stacks for pv domains

On 09/01/18 18:01, Andrew Cooper wrote:
> On 09/01/18 14:27, Juergen Gross wrote:
>> Instead of using the TSS and stacks of the physical processor allocate
>> them per vcpu, map them in the per domain area, and use those.
>>
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>
> I don't see anything here which updates the fields in the TSS across
> context switch.  Without it, you'll be taking NMIs/MCEs/DF's on the
> wrong stack.

No, I'm doing ltr() with a TSS referencing the per-vcpu stacks. TSS is
per vcpu, too.



> I still don't see how your plan is viable in the first place, and is
> adding substantially more complexity to an answer which doesn't need it.
>
> I'm afraid I'm on the verge of a nack unless you can explain how is
> intended to be safe, and better than what we currently have.

It is laying the groundwork for a KAISER solution needing no mapping of
per physical cpu areas in the user guest tables, so isolating the guests
from each other.


Juergen

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