[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH v2] SUPPORT.md: extend security support for x86 hosts to 12 TiB of memory



Hi Jan,

On 25/05/2022 12:11, Jan Beulich wrote:
On 25.05.2022 12:58, Julien Grall wrote:
On 25/05/2022 10:21, Jan Beulich wrote:
--- a/SUPPORT.md
+++ b/SUPPORT.md
@@ -50,7 +50,7 @@ For the Cortex A57 r0p0 - r1p1, see Erra
### Physical Memory - Status, x86: Supported up to 8 TiB. Hosts with more memory are supported, but not security supported.
+    Status, x86: Supported up to 12 TiB. Hosts with more memory are supported, 
but not security supported.
       Status, Arm32: Supported up to 12 GiB
       Status, Arm64: Supported up to 2 TiB
@@ -121,6 +121,17 @@ ARM only has one guest type at the momen Status: Supported +## Guest Limits
+
+### Memory
+
+    Status, x86: Supported up to 8 TiB
+    Status, Arm64: Supported up to 1 TiB
+    Status, Arm32: Supported up to 32 GiB

IIRC, the max the architecture would allow us is 16 Gib. Here we are
limited with how much physical memory is supported by Xen. So this wants
to be 12 GiB.

Hmm, while I don't know where I took the 32 from, it was you who
suggested (in reply to v1) I put 16 here.

Hmmm... I am pretty sure I wrote 16 in v1 [1].

Though yes, with the
host limit now set to just 12, putting more here would be odd.
I didn't cross check the numbers, I'll admit.

+
+Guests with more memory, but less than 16 TiB, are supported,
+but not security supported.

On Arm32, we definitely can't support up to 16 TiB. On Arm64, we would
need some work to support it. So I would move this sentence in the
"Status, x86" section.

Sure, I can do that. Would have been nice if you had said so right
on v1.
I only spotted this oddity now. Sorry.

As to Arm64 though - the host limit is 2 TiB. Going beyond
that being impossible (without becoming at least unsupported), is
the uniform upper bound of 16 TiB really a problem here (IOW do
guests really only function up to 1 TiB)?

The guest memory layout has only been defined for up to 1TB. Hopefully this is the only place where the assumption is baked. But as I don't have such machine in hand, I can't easily confirm it.

So I think we need to clarify that the 16TB limit only applies to x86.

For Arm32 it would be
even less of an issue, as hosts with more than 12 GiB are
unsupported.

This is only obvious if you know that Xen doesn't support memory overcommitting. Admittely, this could be considered as basic Xen knowledge.

Cheers,

[1] https://lore.kernel.org/xen-devel/6ec0e3d9-374c-1caa-9889-f091dcf894e3@xxxxxxx/

--
Julien Grall



 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.