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    SubjectRe: [PATCH v1 4/4] swiotlb: panic if nslabs is too small
    On Mon, Aug 22, 2022 at 4:28 PM Dongli Zhang <dongli.zhang@oracle.com> wrote:
    >
    > Hi Yu, Robin and Christoph,
    >
    > The mips kernel panic because the SWIOTLB buffer is adjusted to a very small
    > value (< 1MB, or < 512-slot), so that the swiotlb panic on purpose.
    >
    > software IO TLB: SWIOTLB bounce buffer size adjusted to 0MB
    > software IO TLB: area num 1.
    > Kernel panic - not syncing: swiotlb_init_remap: nslabs = 128 too small
    >
    >
    > From mips code, the 'swiotlbsize' is set to PAGE_SIZE initially. It is always
    > PAGE_SIZE unless it is used by CONFIG_PCI or CONFIG_USB_OHCI_HCD_PLATFORM.
    >
    > Finally, the swiotlb panic on purpose.
    >
    > 189 void __init plat_swiotlb_setup(void)
    > 190 {
    > ... ...
    > 211 swiotlbsize = PAGE_SIZE;
    > 212
    > 213 #ifdef CONFIG_PCI
    > 214 /*
    > 215 * For OCTEON_DMA_BAR_TYPE_SMALL, size the iotlb at 1/4 memory
    > 216 * size to a maximum of 64MB
    > 217 */
    > 218 if (OCTEON_IS_MODEL(OCTEON_CN31XX)
    > 219 || OCTEON_IS_MODEL(OCTEON_CN38XX_PASS2)) {
    > 220 swiotlbsize = addr_size / 4;
    > 221 if (swiotlbsize > 64 * (1<<20))
    > 222 swiotlbsize = 64 * (1<<20);
    > 223 } else if (max_addr > 0xf0000000ul) {
    > 224 /*
    > 225 * Otherwise only allocate a big iotlb if there is
    > 226 * memory past the BAR1 hole.
    > 227 */
    > 228 swiotlbsize = 64 * (1<<20);
    > 229 }
    > 230 #endif
    > 231 #ifdef CONFIG_USB_OHCI_HCD_PLATFORM
    > 232 /* OCTEON II ohci is only 32-bit. */
    > 233 if (OCTEON_IS_OCTEON2() && max_addr >= 0x100000000ul)
    > 234 swiotlbsize = 64 * (1<<20);
    > 235 #endif
    > 236
    > 237 swiotlb_adjust_size(swiotlbsize);
    > 238 swiotlb_init(true, SWIOTLB_VERBOSE);
    > 239 }
    >
    >
    > Here are some thoughts. Would you mind suggesting which is the right way to go?
    >
    > 1. Will the PAGE_SIZE swiotlb be used by mips when it is only PAGE_SIZE? If it
    > is not used, why not disable swiotlb completely in the code?
    >
    > 2. The swiotlb panic on purpose when it is less then 1MB. Should we remove that
    > limitation?
    >
    > 3. ... or explicitly declare the limitation that: "swiotlb should be at least
    > 1MB, otherwise please do not use it"?
    >
    >
    > The reason I add the panic on purpose is for below case:
    >
    > The user's kernel is configured with very small swiotlb buffer. As a result, the
    > device driver may work abnormally.

    Which driver? This sounds like that driver is broken, and we should
    fix that driver.

    > As a result, the issue is reported to a
    > specific driver's developers, who spend some time to confirm it is swiotlb
    > issue.

    Is this a fact or a hypothetical proposition?

    > Suppose those developers are not familiar with IOMMU/swiotlb, it takes
    > times until the root cause is identified.

    Sorry but you are making quite a few assumptions in a series claimed
    to be "swiotlb: some cleanup" -- I personally expect cleanup patches
    not to have any runtime side effects.

    > If we panic earlier, the issue will be reported to IOMMU/swiotlb developer.

    Ok, I think we should at least revert this patch, if not the entire series.

    > This
    > is also to sync with the remap failure logic in swiotlb (used by xen).

    We can have it back in after we have better understood how it
    interacts with different archs/drivers, or better yet when the needs
    arise, if they arise at all.

    Thanks.

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