[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] an assertion triggered when running Xen on a HSW desktop
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. The other option as said above is to force the caller to then have a loop that performs a bunch of order 0 guest_physmap_add_page until it reaches a 2MB aligned address. > 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. Thanks, Roger. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel
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