Re: [kvm-unit-tests PATCH v3 3/6] s390x: sie: switch to home space mode before entering SIE

Nico Boehr
2023-06-15T13:09:17Z

Quoting Janosch Frank (2023-06-05 11:03:19)
> On 6/1/23 09:01, Nico Boehr wrote:
> > This is to prepare for running guests without MSO/MSL, which is
> > currently not possible.
>
> This is a preparation patch for non-MSO/MSL guest support.
>
> >
> > We already have code in sie64a to setup a guest primary ASCE before
> > entering SIE, so we can in theory switch to the page tables which
> > translate gpa to hpa.
>
> The important information that's missing here is that SIE always uses
> the prim space for guest to host translations.
>
> >
> > But the host is running in primary space mode already, so changing the
> > primary ASCE before entering SIE will also affect the host's code and
> > data.
>
> And that's why this is an issue.
>
> For the time being we'll copy the primary ASCE into the home space ASCE.
> No functional change is intended. But if a test intends to....

Thanks, I have to following text now:

s390x: sie: switch to home space mode before entering SIE

This is a preparation patch for running guests without MSO/MSL.

We already have code in sie64a to setup a guest primary ASCE before
entering SIE. Since SIE always uses the primary ASCE to translate gpa to
hpa we can in theory switch to seperate guest page tables.

But the host is running in primary space mode already, so changing the
primary ASCE before entering SIE will also affect the host's code and
data. That's why running the host in primary space is an issue.

To make this switch useful, the host should run in a different address
space mode. Hence, set up and change to home address space mode before
installing the guest ASCE.

For the time being the primary ASCE is copied to the home space ASCE.
No functional change is intended, but if a test intends to use a
different primary space ASCE, it can now just set the guest.asce in the
save_area.

> > diff --git a/lib/s390x/sie.c b/lib/s390x/sie.c
> > index 9241b4b4a512..ffa8ec91a423 100644
> > --- a/lib/s390x/sie.c
> > +++ b/lib/s390x/sie.c
> > @@ -46,6 +46,8 @@ void sie_handle_validity(struct vm *vm)
> >
> > void sie(struct vm *vm)
> > {
[...]
> > + /* switch to home space so guest tables can be different from host */
> > + psw_mask_set_bits(PSW_MASK_HOME);
> > +
> > + /* also handle all interruptions in home space while in SIE */
> > + irq_set_dat_mode(IRQ_DAT_ON, AS_HOME);
>
> I'm wondering why this needs to be two calls when you clearly want to
> have a convenience function that does a full space change.
>
> We could introduce:
>
> #define AS_REAL 4
>
> static void mmu_set_addr_space(uint8_t space)
> {
> if (space < 4)
> psw_mask_set_bits(space);
> irq_set_dat_mode(space);
> }
>
> The "addr" in the function name is optional in my opinion.
> @Claudio: What's your opinion?

I don't agree; this convenience function would just hide your intention.

With the code right now it is perfectly clear that we're running in home and
handling all interruptions in home. It is perfectly valid to handle
interruptions in a space different from the one you're currently running in;
I guess that's what every OS does for userspace ;-)