Re: [PATCH v2 4/7] s390/physmem_info: query diag500(STORAGE LIMIT) to support QEMU/KVM memory devices

[David Hildenbrand]

On 14.10.24 20:43, Heiko Carstens wrote:
> On Mon, Oct 14, 2024 at 04:46:16PM +0200, David Hildenbrand wrote:
> > To support memory devices under QEMU/KVM, such as virtio-mem,
> > we have to prepare our kernel virtual address space accordingly and
> > have to know the highest possible physical memory address we might see
> > later: the storage limit. The good old SCLP interface is not suitable for
> > this use case.
> >
> > In particular, memory owned by memory devices has no relationship to
> > storage increments, it is always detected using the device driver, and
> > unaware OSes (no driver) must never try making use of that memory.
> > Consequently this memory is located outside of the "maximum storage
> > increment"-indicated memory range.
> >
> > Let's use our new diag500 STORAGE_LIMIT subcode to query this storage
> > limit that can exceed the "maximum storage increment", and use the
> > existing interfaces (i.e., SCLP) to obtain information about the initial
> > memory that is not owned+managed by memory devices.
> >
> > If a hypervisor does not support such memory devices, the address exposed
> > through diag500 STORAGE_LIMIT will correspond to the maximum storage
> > increment exposed through SCLP.
> >
> > To teach kdump on s390 to include memory owned by memory devices, there
> > will be ways to query the relevant memory ranges from the device via a
> > driver running in special kdump mode (like virtio-mem already implements
> > to filter /proc/vmcore access so we don't end up reading from unplugged
> > device blocks).
> >
> > Tested-by: Mario Casquero <mcasquer@xxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >   arch/s390/boot/physmem_info.c        | 46 ++++++++++++++++++++++++++--
> >   arch/s390/include/asm/physmem_info.h |  3 ++
> >   2 files changed, 46 insertions(+), 3 deletions(-)
>
> ...
>
> > +static int diag500_storage_limit(unsigned long *max_physmem_end)
> > +{
> > +	register unsigned long __nr asm("1") = 0x4;
> > +	register unsigned long __storage_limit asm("2") = 0;
> > +	unsigned long reg1, reg2;
> > +	psw_t old;
>
> In general we do not allow register asm usage anymore in s390 code,
> except for a very few defined places. This is due to all the problems
> that we've seen with code instrumentation and register corruption.

Makes sense. Note that I was inspired by GENERATE_KVM_HYPERCALL_FUNC 
that also still uses register asm usage.

> The patch below changes your code accordingly, but it is
> untested. Please verify that your code still works.

Below LGTM, I'll give it a churn, thanks!

--
Cheers,

David / dhildenb