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

[Tim Deegan <tim@xen.org>]

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.

Cheers,

Tim.