Re: [PATCH] Updates to Xen hypercall preemption

[Andrew Cooper]

On 22/06/2023 9:26 am, Peter Zijlstra wrote:
> On Thu, Jun 22, 2023 at 07:22:53AM +0200, Juergen Gross wrote:
>
>> The hypercalls we are talking of are synchronous ones. They are running
>> in the context of the vcpu doing the call (like a syscall from userland is
>> running in the process context).
> (so time actually passes from the guest's pov?)

Yes.  And in principle it's wired into stolen time.

>> The hypervisor will return to guest context from time to time by modifying
>> the registers such that the guest will do the hypercall again with different
>> input values for the hypervisor, resulting in a proper continuation of the
>> hypercall processing.
> Eeeuw.. that's pretty terrible. And changing this isn't in the cards,
> like at all?
>
> That is, why isn't this whole thing written like:
>
> 	for (;;) {
> 		ret = hypercall(foo);
> 		if (ret == -EAGAIN) {
> 			cond_resched();
> 			continue;
> 		}
> 		break;
> 	}

No, because that would have required the original authors to write a
coherent interface.

As example, I present "long hypercall_get_dr(int reg);" which returns a
full %dr, or -EINVAL.  But other hypercalls have completely disjoint
API/ABIs so even if you could tell Xen not subtract %rip to repeat the
SYSCALL/etc instruction, you still don't have any kind of way to spot a
continuation.

Making a new ABI/API which looks like that is specifically on the cards,
in some copious free time.

~Andrew