Re: [PATCH v2 3/5] hw/arm/virt: Honor highmem setting when computing the memory map

[Marc Zyngier]

On Mon, 04 Oct 2021 13:23:41 +0100,
Eric Auger <eric.auger@redhat.com> wrote:
> 
> Hi Marc,
> 
> On 10/3/21 6:46 PM, Marc Zyngier wrote:
> > Even when the VM is configured with highmem=off, the highest_gpa
> > field includes devices that are above the 4GiB limit.
> > Similarily, nothing seem to check that the memory is within
> > the limit set by the highmem=off option.
> >
> > This leads to failures in virt_kvm_type() on systems that have
> > a crippled IPA range, as the reported IPA space is larger than
> > what it should be.
> >
> > Instead, honor the user-specified limit to only use the devices
> > at the lowest end of the spectrum, and fail if we have memory
> > crossing the 4GiB limit.
> >
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  hw/arm/virt.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> > index bcf58f677d..9d2abdbd5f 100644
> > --- a/hw/arm/virt.c
> > +++ b/hw/arm/virt.c
> > @@ -1628,6 +1628,11 @@ static void virt_set_memmap(VirtMachineState *vms)
> >          exit(EXIT_FAILURE);
> >      }
> >  
> > +    if (!vms->highmem &&
> > +        vms->memmap[VIRT_MEM].base + ms->maxram_size > 4 * GiB) {
> > +        error_report("highmem=off, but memory crosses the 4GiB limit\n");
> > +        exit(EXIT_FAILURE);
> > +    }
> >      /*
> >       * We compute the base of the high IO region depending on the
> >       * amount of initial and device memory. The device memory start/size
> > @@ -1657,7 +1662,9 @@ static void virt_set_memmap(VirtMachineState *vms)
> >          vms->memmap[i].size = size;
> >          base += size;
> >      }
> > -    vms->highest_gpa = base - 1;
> > +    vms->highest_gpa = (vms->highmem ?
> > +                        base :
> > +                        vms->memmap[VIRT_MEM].base + ms->maxram_size) - 1;
> I think I would have preferred to have
> 
> if (vms->highmem) {
>    for (i = VIRT_LOWMEMMAP_LAST; i < ARRAY_SIZE(extended_memmap); i++) {
>         hwaddr size = extended_memmap[i].size;
> 
>         base = ROUND_UP(base, size);
>         vms->memmap[i].base = base;
>         vms->memmap[i].size = size;
>         base += size;
>     }
> }
> as it is useless to execute that code and create new memmap entries in
> case of !highmem.

I agree that it is a bit useless when we only have highmem. But we
really want to deal with arbitrary IPA spaces (see how this changes in
the follow-up patches), and we need to check that everything fits in
the IPA space (and fix things up if they don't).

> 
> But nevertheless, this looks correct

Thanks,

        M.

-- 
Without deviation from the norm, progress is not possible.