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Re: [PATCH v4 03/10] tools/libxl: add vmtrace_pt_size parameter


On 06/07/2020 09:46, Jan Beulich wrote:
On 04.07.2020 19:23, Julien Grall wrote:

On 03/07/2020 11:11, Roger Pau Monné wrote:
On Fri, Jul 03, 2020 at 11:56:38AM +0200, Jan Beulich wrote:
On 03.07.2020 11:44, Roger Pau Monné wrote:
On Thu, Jul 02, 2020 at 06:23:28PM +0200, Michał Leszczyński wrote:
----- 2 lip 2020 o 11:00, Roger Pau Monné roger.pau@xxxxxxxxxx napisał(a):

On Tue, Jun 30, 2020 at 02:33:46PM +0200, Michał Leszczyński wrote:
diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 59bdc28c89..7b8289d436 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -92,6 +92,7 @@ struct xen_domctl_createdomain {
       uint32_t max_evtchn_port;
       int32_t max_grant_frames;
       int32_t max_maptrack_frames;
+    uint8_t vmtrace_pt_order;

I've been thinking about this, and even though this is a domctl (so
not a stable interface) we might want to consider using a size (or a
number of pages) here rather than an order. IPT also supports
TOPA mode (kind of a linked list of buffers) that would allow for
sizes not rounded to order boundaries to be used, since then only each
item in the linked list needs to be rounded to an order boundary, so
you could for example use three 4K pages in TOPA mode AFAICT.


In previous versions it was "size" but it was requested to change it
to "order" in order to shrink the variable size from uint64_t to
uint8_t, because there is limited space for xen_domctl_createdomain

It's likely I'm missing something here, but I wasn't aware
xen_domctl_createdomain had any constrains regarding it's size. It's
currently 48bytes which seems fairly small.

Additionally I would guess a uint32_t could do here, if the value
passed was "number of pages" rather than "number of bytes"?
Looking at the rest of the code, the toolstack accepts a 64-bit value.
So this would lead to truncation of the buffer if it is bigger than 2^44

I agree such buffer is unlikely, yet I still think we want to harden the
code whenever we can. So the solution is to either prevent check
truncation in libxl or directly use 64-bit in the domctl.

My preference is the latter.

That could work, not sure if it needs to state however that those will
be 4K pages, since Arm can have a different minimum page size IIRC?
(or that's already the assumption for all number of frames fields)
vmtrace_nr_frames seems fine to me.

The hypercalls interface is using the same page granularity as the
hypervisor (i.e 4KB).

While we already support guest using 64KB page granularity, it is
impossible to have a 64KB Arm hypervisor in the current state. You are
going to either break existing guest (if you switch to 64KB page
granularity for the hypercall ABI) or render them insecure (the mimimum
mapping in the P2M would be 64KB).

DOMCTLs are not stable yet, so using a number of pages is OK. However, I
would strongly suggest to use a number of bytes for any xl/libxl/stable
libraries interfaces as this avoids confusion and also make more

If we can't settle on what "page size" means in the public interface
(which imo is embarrassing), then how about going with number of kb,
like other memory libxl controls do? (I guess using Mb, in line with
other config file controls, may end up being too coarse here.) This
would likely still allow for a 32-bit field to be wide enough.

A 32-bit field would definitely not be able to cover a full address space. So do you mind to explain what is the upper bound you expect here?


Julien Grall



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