Re: kvm vmload/vmsave vs tss.ist

[Avi Kivity <avi@redhat.com>]

Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> wrote:
>
>   
>> i'd suggest to reuse the irq-stacks for this. Right now on 64-bit we've 
>> got the following stack layout: 8K process stacks, a 16K IRQ stack on 
>> each CPU, shared by all IRQs. Then we have the IST stacks with weird 
>> sizes: debug:8K, the others: 4K.
>>     
>
> this has to be done carefully though, as there's a subtle detail here: 
> right now the pda_irqcount and the pda_irqstackptr logic in entry_64.S is 
> not re-entry safe and relies on IRQs being off.
>
> If critical exceptions are moved to the IRQ stack then %rsp switching to 
> the IRQ stack has to be done atomically: instead of using the pda_irqcount 
> check the %rsp value itself should be checked against pda_irqstackptr - if 
> it's within that 16K range then we are already on the IRQ stack and do not 
> need to switch to it but can just use the current %rsp.
>   

I think it's enough to switch %rsp before incrementing irqcount, no?

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.