Messages in this thread | | | Subject | Re: [PATCH] xen-swiotlb: exchange memory with Xen only when pages are contiguous | From | Boris Ostrovsky <> | Date | Thu, 25 Oct 2018 12:10:13 -0400 |
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On 10/25/18 10:23 AM, Joe Jin wrote: > On 10/25/18 4:45 AM, Boris Ostrovsky wrote: >> On 10/24/18 10:43 AM, Joe Jin wrote: >>> On 10/24/18 6:57 AM, Boris Ostrovsky wrote: >>>> On 10/24/18 9:02 AM, Konrad Rzeszutek Wilk wrote: >>>>> On Tue, Oct 23, 2018 at 08:09:04PM -0700, Joe Jin wrote: >>>>>> Commit 4855c92dbb7 "xen-swiotlb: fix the check condition for >>>>>> xen_swiotlb_free_coherent" only fixed memory address check condition >>>>>> on xen_swiotlb_free_coherent(), when memory was not physically >>>>>> contiguous and tried to exchanged with Xen via >>>>>> xen_destroy_contiguous_region it will lead kernel panic. >>>>> s/it will lead/which lead to/? >>>>> >>>>>> The correct check condition should be memory is in DMA area and >>>>>> physically contiguous. >>>>> "The correct check condition to make Xen hypercall to revert the >>>>> memory back from its 32-bit pool is if it is: >>>>> 1) Above its DMA bit mask (for example 32-bit devices can only address >>>>> up to 4GB, and we may want 4GB+2K), and >>>> Is this "and' or 'or'? >>>> >>>>> 2) If it not physically contingous >>>>> >>>>> N.B. The logic in the code is inverted, which leads to all sorts of >>>>> confusions." >>>> I would, in fact, suggest to make the logic the same in both >>>> xen_swiotlb_alloc_coherent() and xen_swiotlb_free_coherent() to avoid >>>> this. This will involve swapping if and else in the former. >>>> >>>> >>>>> Does that sound correct? >>>>> >>>>>> Thank you Boris for pointing it out. >>>>>> >>>>> Fixes: 4855c92dbb7 ("xen-sw..") ? >>>>> >>>>>> Signed-off-by: Joe Jin <joe.jin@oracle.com> >>>>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> >>>>>> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> >>>>> Reported-by: Boris Ostrovs... ? >>>>>> Cc: Christoph Helwig <hch@lst.de> >>>>>> Cc: Dongli Zhang <dongli.zhang@oracle.com> >>>>>> Cc: John Sobecki <john.sobecki@oracle.com> >>>>>> --- >>>>>> drivers/xen/swiotlb-xen.c | 4 ++-- >>>>>> 1 file changed, 2 insertions(+), 2 deletions(-) >>>>>> >>>>>> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c >>>>>> index f5c1af4ce9ab..aed92fa019f9 100644 >>>>>> --- a/drivers/xen/swiotlb-xen.c >>>>>> +++ b/drivers/xen/swiotlb-xen.c >>>>>> @@ -357,8 +357,8 @@ xen_swiotlb_free_coherent(struct device *hwdev, size_t size, void *vaddr, >>>>>> /* Convert the size to actually allocated. */ >>>>>> size = 1UL << (order + XEN_PAGE_SHIFT); >>>>>> >>>>>> - if (((dev_addr + size - 1 <= dma_mask)) || >>>>>> - range_straddles_page_boundary(phys, size)) >>>>>> + if ((dev_addr + size - 1 <= dma_mask) && >>>>>> + !range_straddles_page_boundary(phys, size)) >>>>>> xen_destroy_contiguous_region(phys, order); >>>> I don't think this is right. >>>> >>>> if ((dev_addr + size - 1 > dma_mask) || range_straddles_page_boundary(phys, size)) >>>> >>>> No? >>> No this is not correct. >>> >>> When allocate memory, it tried to allocated from Dom0/Guest, then check if physical >>> address is DMA memory also contiguous, if no, exchange with Hypervisor, code as below: >>> >>> 326 phys = *dma_handle; >>> 327 dev_addr = xen_phys_to_bus(phys); >>> 328 if (((dev_addr + size - 1 <= dma_mask)) && >>> 329 !range_straddles_page_boundary(phys, size)) >>> 330 *dma_handle = dev_addr; >>> 331 else { >>> 332 if (xen_create_contiguous_region(phys, order, >>> 333 fls64(dma_mask), dma_handle) != 0) { >>> 334 xen_free_coherent_pages(hwdev, size, ret, (dma_addr_t)phys, attrs); >>> 335 return NULL; >>> 336 } >>> 337 } >>> >>> >>> On freeing, need to return the memory to Xen, otherwise DMA memory will be used >>> up(this is the issue the patch intend to fix), so when memory is DMAable and >>> contiguous then call xen_destroy_contiguous_region(), return DMA memory to Xen. >> So if you want to allocate 1 byte at address 0 (and dev_addr=phys), >> xen_create_contiguous_region() will not be called. And yet you will call >> xen_destroy_contiguous_region() in the free path. >> >> Is this the expected behavior? > I could not say it's expected behavior, but I think it's reasonable.
I would expect xen_create_contiguous_region() and xen_destroy_contiguous_region() to come in pairs. If a region is created, it needs to be destroyed. And vice versa.
> > On allocating, it used __get_free_pages() to allocate memory, if lucky the memory is > DMAable, will not exchange memory with hypervisor, obviously this is not guaranteed. > > And on freeing it could not be identified if memory from Dom0/guest own memory > or hypervisor
I think it can be. if (!(dev_addr + size - 1 <= dma_mask) || range_straddles_page_boundary()) then it must have come from the hypervisor, because that's the check we make in xen_swiotlb_alloc_coherent().
-boris
> , if don't back memory to hypervisor which will lead hypervisor DMA > memory be used up, then on Dom0/guest, DMA request maybe failed, the worse thing is > could not start any new guest. > > Thanks, > Joe > >> -boris >>
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