Re: [Xen-devel] [PATCH v4 10/23] xen/arm: introduce allocate_memory

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Sat, 20 Oct 2018, Julien Grall wrote:
> > > > +
> > > > +    pg = alloc_domheap_pages(d, order, 0);
> > > 
> > > So here you impose the memory to be contiguously allocated for a given
> > > bank.
> > > There are quite a few case where you may not have enough memory to
> > > allocate
> > > contiguously.
> > > 
> > > So more likely you want to add loop in this code to allocate until order
> > > is
> > > reached.
> > 
> > This case is not handled today for dom0.
> 
> I am afraid this is slightly untrue. Dom0 memory is direct mapped, Xen will
> first try to allocate the using the biggest order. If it fails, the order will
> be decreased until the size is suitable. We will always try to allocate the
> maximum of memory.
> 
> There are three major problems with your code:
>       1) There may not be enough contiguous space in the host memory for a
> 4GB region.
>       2) The memory specify by the user may not be in power of 2 pages. For
> instance, a user can specify 40KB. With your algo, we will end to allocate
> 32KB in the first bank and 8KB in the second bank. However what we want is
> allocate 40KB in a single bank.
>       3) You don't check whether the leftover memory is bigger than the size
> of the second bank.
> 
> In the case where the memory is not direct mapped, we should be able to
> allocate the whole region with multiple non-contiguous memory. I think the
> algorithm in used in libxc (see populate_guest_memory) is working quite well
> for this purpose.
> 
> It is also possible to have a simpler solution within the hypervisor. I
> quickly wrote a patch (not compiled, nor tested) that should address the 3
> problems above. See: https://pastebin.com/FQ9k9CbL

Thank you for your help. Actually I had already written something
along the same lines, I don't know why I didn't say it in my reply... I
double checked the implementation with yours here and they look
equivalent. Good.

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