Re: [Xen-devel] an assertion triggered when running Xen on a HSW desktop

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.01.19 at 12:07, <roger.pau@xxxxxxxxxx> wrote:
> On Tue, Jan 15, 2019 at 03:49:07AM -0700, Jan Beulich wrote:
>> >>> On 15.01.19 at 11:27, <roger.pau@xxxxxxxxxx> wrote:
>> > On Tue, Jan 15, 2019 at 03:16:01AM -0700, Jan Beulich wrote:
>> >> >>> On 15.01.19 at 10:44, <Paul.Durrant@xxxxxxxxxx> wrote:
>> >> >>  -----Original Message-----
>> >> > [snip]
>> >> >> >> (XEN) Xen call trace:
>> >> >> >> (XEN)    [<ffff82d08025ccbc>] iommu_map+0xba/0x176
>> >> >> >> (XEN)    [<ffff82d0804182d8>] iommu_hwdom_init+0xef/0x220
>> >> >> >> (XEN)    [<ffff82d08043716c>] dom0_construct_pvh+0x189/0x129e
>> >> >> >> (XEN)    [<ffff82d08043e53c>] construct_dom0+0xd4/0xb14
>> >> >> >> (XEN)    [<ffff82d08042d8ef>] __start_xen+0x2710/0x2830
>> >> >> >> (XEN)    [<ffff82d0802000f3>] __high_start+0x53/0x55
>> >> >> >> (XEN)
>> >> >> >> (XEN)
>> >> >> >> (XEN) ****************************************
>> >> >> >> (XEN) Panic on CPU 0:
>> >> >> >> (XEN) Assertion 'IS_ALIGNED(dfn_x(dfn), (1ul << page_order))' 
>> >> >> >> failed at
>> >> >> iommu.c:323
>> >> >> >> (XEN) ****************************************
>> >> >> >
>> >> >> >Oh, this was added by Paul quite recently. You seem to be using a
>> >> >> >rather old commit (a5b0eb3636), is there any reason for using such an
>> >> >> >old baseline?
>> >> >> 
>> >> >> I was using the master branch. Your patch below did fix this issue.
>> >> > 
>> >> > Given this failure and the fact that valid orders differ between 
>> >> > different 
> 
>> >> > architectures, I propose we change the argument to the iommu_map/unmap 
>> >> > wrapper functions from an order to a count, thus making it clear that 
> there 
>> > 
>> >> > is no alignment restriction.
>> >> 
>> >> But the whole idea is for there to be an alignment restriction, such
>> >> that it is easy to determine whether large page mappings can be
>> >> used to satisfy the request. What's the exact case where a caller
>> >> absolutely has to pass in a mis-aligned (dfn,size) tuple?
>> > 
>> > Taking PVH Dom0 builder as an example, it's possible to have a RAM
>> > region that starts on a 4K only aligned address. The natural operation
>> > in that case would be to try to allocate a memory region as big as
>> > possible up to the next 2MB boundary. Hence it would be valid to
>> > attempt to populate this 4K only aligned address using an order > 0
>> > and < 9 (2MB order). The alternative here if the asserts are not
>> > removed would be to open-code a loop in the caller that iterates
>> > creating a bunch of order 0 mappings up to the 2MB boundary. The
>> > overhead in that case would be quite big, so I don't think we want to
>> > go down that route (also we would end up with a bunch of loops in the
>> > callers).
>> 
>> I'm afraid I'm now more confused than before: If there's a RAM
>> region aligned to no better than 4k, how can this possibly be
>> populated with an order-greater-than-zero allocation?
> 
> Why not? You can request a memory chunk of order 5 from
> alloc_domheap_pages for example and pass that to
> guest_physmap_add_page. That would be a perfectly fine operation to do
> in order to reach a memory address that's aligned to a 2MB boundary.

I think it is never a good idea to request chunks of larger
alignment (and hence higher order) than is necessary.

I still don't understand the second sentence in this context though,
so I guess I must still be missing something.

>> And even
>> if I re-phrased your reply to mean an arbitrary alignment / order
>> less than 9, then populating this with such a smaller order is still
>> fine, and requesting the IOMMU mapping with that smaller order
>> is still not going to trip the ASSERT() in question.
> 
> But the caller is then forced to iterate over the region and populate
> it with order 0 calls to guest_physmap_add_page, which introduces a
> lot of overhead.

How is the overhead going to be any smaller if it's not the caller
but iommu_map() to do the looping?

Jan



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