Re: [Xen-devel] [PATCH v3 3/3] tools: introduce parameter max_wp_ram_ranges.

[George Dunlap <dunlapg@xxxxxxxxx>]

On Thu, Feb 4, 2016 at 9:38 AM, Yu, Zhang <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> On 2/4/2016 5:28 PM, Paul Durrant wrote:
>> I assume this means that the emulator can 'unshadow' GTTs (I guess on an
>> LRU basis) so that it can shadow new ones when the limit has been exhausted?
>> If so, how bad is performance likely to be if we live with a lower limit
>> and take the hit of unshadowing if the guest GTTs become heavily fragmented?
>>
> Thank you, Paul.
>
> Well, I was told the emulator have approaches to delay the shadowing of
> the GTT till future GPU commands are submitted. By now, I'm not sure
> about the performance penalties if the limit is set too low. Although
> we are confident 8K is a secure limit, it seems still too high to be
> accepted. We will perform more experiments with this new approach to
> find a balance between the lowest limit and the XenGT performance.

Just to check some of my assumptions:

I assume that unlike memory accesses, your GPU hardware cannot
'recover' from faults in the GTTs. That is, for memory, you can take a
page fault, fix up the pagetables, and then re-execute the original
instruction; but so far I haven't heard of any devices being able to
seamlessly re-execute a transaction after a fault.  Is my
understanding correct?

If that is the case, then for every top-level value (whatever the
equivalent of the CR3), you need to be able to shadow the entire GTT
tree below it, yes?  You can't use a trick that the memory shadow
pagetables can use, of unshadowing parts of the tree and reshadowing
them.

So as long as the currently-in-use GTT tree contains no more than
$LIMIT ranges, you can unshadow and reshadow; this will be slow, but
strictly speaking correct.

What do you do if the guest driver switches to a GTT such that the
entire tree takes up more than $LIMIT entries?

 -George

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