Re: [RFC 3/4] HVM x86 deprivileged mode: Code for switching into/out of deprivileged mode

[Ben Catterall <Ben.Catterall@citrix.com>]

On 10/08/15 10:49, Tim Deegan wrote:
> Hi,
>
> At 17:45 +0100 on 06 Aug (1438883118), Ben Catterall wrote:
>> The process to switch into and out of deprivileged mode can be likened to
>> setjmp/longjmp.
>>
>> To enter deprivileged mode, we take a copy of the stack from the guest's
>> registers up to the current stack pointer.
>
> This copy is pretty unfortunate, but I can see that avoiding it will
> be a bit complex.  Could we do something with more stacks?  AFAICS
> there have to be three stacks anyway:
>
>   - one to hold the depriv execution context;
>   - one to hold the privileged execution context; and
>   - one to take interrupts on.
>
> So maybe we could do some fiddling to make Xen take interrupts on a
> different stack while we're depriv'd?
>
> If we do have to copy, we could track whether the original stack has
> been clobbered by an interrupt, and so avoid (at least some of) the
> copy back afterwards?
>
> One nit in the assembler - if I've followed correctly, this saved IP:
>
>> +        /* Perform a near call to push rip onto the stack */
>> +        call   1f
>
> is returned to (with adjustments) here:
>
>> +        /* Go to user mode return code */
>> +        jmp    *(%rsi)
>
> It would be good to make this a matched pair of call/ret if we can;
> the CPU has special branch prediction tracking for function calls that
> gets confused by a call that's not returned to.
>
sure, will do.
> Cheers,
>
> Tim.
>