From: Heiko Carstens <hca@linux.ibm.com>
To: David Hildenbrand <david@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>,
Janosch Frank <frankja@linux.ibm.com>,
"Michael S . Tsirkin" <mst@redhat.com>,
Cornelia Huck <cohuck@redhat.com>,
qemu-devel@nongnu.org, Halil Pasic <pasic@linux.ibm.com>,
Christian Borntraeger <borntraeger@de.ibm.com>,
qemu-s390x@nongnu.org, Claudio Imbrenda <imbrenda@linux.ibm.com>,
Richard Henderson <rth@twiddle.net>
Subject: Re: [PATCH RFC 2/5] s390x: implement diag260
Date: Wed, 15 Jul 2020 12:43:48 +0200 [thread overview]
Message-ID: <20200715104348.GB6927@osiris> (raw)
In-Reply-To: <a0b52f65-d253-c51b-6dfc-0a61f939c759@redhat.com>
On Wed, Jul 15, 2020 at 11:42:37AM +0200, David Hildenbrand wrote:
> So, in summary, we want to indicate to the guest a memory region that
> will be used to place memory devices ("device memory region"). The
> region might have holes and the memory within this region might have
> different semantics than ordinary system memory. Memory that belongs to
> memory devices should only be detected+used if the guest OS has support
> for them (e.g., virtio-mem, virtio-pmem, ...). An unmodified guest
> (e.g., no virtio-mem driver) should not accidentally make use of such
> memory.
>
> We need a way to
> a) Tell the guest about boot memory (currently ram_size)
> b) Tell the guest about the maximum possible ram address, including
> device memory. (We could also indicate the special "device memory
> region" explicitly)
>
> AFAIK, we have three options:
>
> 1. Indicate maxram_size via SCLP, indicate ram_size via diag260(0x10)
>
> This is what this series (RFCv1 does).
>
> Advantages:
> - No need for a new diag. No need for memory sensing kernel changes.
> Disadvantages
> - Older guests without support for diag260 (<v4.2, kvm-unit-tests) will
> assume all memory is accessible. Bad.
Why would old guests assume that?
At least in v4.1 the kernel will calculate the max address by using
increment size * increment number and then test if *each* increment is
available with tprot.
> - The semantics of the value returned in ry via diag260(0xc) is somewhat
> unclear. Should we return the end address of the highest memory
> device? OTOH, an unmodified guest OS (without support for memory
> devices) should not have to care at all about any such memory.
I'm confused. The kernel currently only uses diag260(0x10). How is
diag260(0xc) relevant here?
> 3. Indicate maxram_size and ram_size via SCLP (using the SCLP standby
> memory)
>
> I did not look into the details, because -ENODOCUMENTATION. At least we
> would run into some alignment issues (again, having to align
> ram_size/maxram_size to storage increments - which would no longer be
> 1MB). We would run into issues later, trying to also support standby memory.
That doesn't make sense to me: either support memory hotplug via
sclp/standby memory, or with your new method. But trying to support
both.. what's the use case?
next prev parent reply other threads:[~2020-07-15 10:44 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-08 18:51 [PATCH RFC 0/5] s390x: initial support for virtio-mem David Hildenbrand
2020-07-08 18:51 ` [PATCH RFC 1/5] s390x: move setting of maximum ram size to machine init David Hildenbrand
2020-07-08 18:51 ` [PATCH RFC 2/5] s390x: implement diag260 David Hildenbrand
2020-07-09 10:37 ` Cornelia Huck
2020-07-09 17:54 ` David Hildenbrand
2020-07-10 8:32 ` David Hildenbrand
2020-07-10 8:41 ` David Hildenbrand
2020-07-10 9:19 ` Cornelia Huck
2020-07-13 11:54 ` Christian Borntraeger
2020-07-13 12:11 ` Cornelia Huck
2020-07-13 12:13 ` Christian Borntraeger
2020-07-09 10:52 ` Christian Borntraeger
2020-07-09 18:15 ` David Hildenbrand
2020-07-10 9:17 ` David Hildenbrand
2020-07-10 12:12 ` David Hildenbrand
2020-07-10 15:18 ` Heiko Carstens
2020-07-10 15:24 ` David Hildenbrand
2020-07-10 15:43 ` Heiko Carstens
2020-07-10 15:45 ` David Hildenbrand
2020-07-13 9:12 ` Heiko Carstens
2020-07-13 10:27 ` David Hildenbrand
2020-07-13 11:08 ` Christian Borntraeger
2020-07-15 9:42 ` David Hildenbrand
2020-07-15 10:43 ` Heiko Carstens [this message]
2020-07-15 11:21 ` David Hildenbrand
2020-07-15 11:34 ` Heiko Carstens
2020-07-15 11:42 ` David Hildenbrand
2020-07-15 16:14 ` Heiko Carstens
2020-07-15 17:38 ` David Hildenbrand
2020-07-15 17:51 ` David Hildenbrand
2020-07-20 14:43 ` Heiko Carstens
2020-07-20 15:43 ` David Hildenbrand
2020-07-08 18:51 ` [PATCH RFC 3/5] s390x: prepare device memory address space David Hildenbrand
2020-07-09 10:59 ` Cornelia Huck
2020-07-10 7:46 ` David Hildenbrand
2020-07-08 18:51 ` [PATCH RFC 4/5] s390x: implement virtio-mem-ccw David Hildenbrand
2020-07-09 9:24 ` Cornelia Huck
2020-07-09 9:26 ` David Hildenbrand
2020-07-08 18:51 ` [PATCH RFC 5/5] s390x: initial support for virtio-mem David Hildenbrand
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