linux-kernel - Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not enough ram for it

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Date:	Mon, 7 Jan 2013 12:32:31 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Shuah Khan <shuahkhan@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>, Jan Kiszka <jan.kiszka@....de>,
	Jason Wessel <jason.wessel@...driver.com>,
	linux-kernel@...r.kernel.org, Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not
 enough ram for it

On Mon, Jan 7, 2013 at 7:26 AM, Konrad Rzeszutek Wilk
<konrad.wilk@...cle.com> wrote:
> On Fri, Jan 04, 2013 at 02:10:25PM -0800, Yinghai Lu wrote:
>> On Fri, Jan 4, 2013 at 1:02 PM, Shuah Khan <shuahkhan@...il.com> wrote:
>> > Pani'cing the system doesn't sound like a good option to me in this
>> > case. This change to disable swiotlb is made for kdump. However, with
>> > this change several system fail to boot, unless crashkernel_low=72M is
>> > specified.
>>
>> this patchset is new feature to put second kdump kernel above 4G.
>>
>> >
>> > I would the say the right approach to solve this would be to not
>> > change the current pci_swiotlb_detect_override() behavior and treat
>> > swiotlb =1 upon entry equivalent to swiotlb_force set.
>>
>> that will make intel system have to take crashkernel_low=72M too.
>> otherwise intel system will get panic during swiotlb allocation.
>
> Two things:
>
>  1). You need to wrap the 'is_enough_..' in CONFIG_KEXEC, which means
>     that the function needs to go in a header file.

kdump is just one case, user still can boot the system with memmap=...

to hit the panic.

>  2). The check for 1MB is suspect. Why only 1MB? You mentioned it is
>      b/c of crashkernel_low=72M (which I am not seeing in v3.8 kernel-parameters.txt?
>      Is that part of your mega-patchset?). Anyhow, there seems to be a disconnect -
>      what if the user supplied crashkernel_low=27M? Perhaps the 'is_enough'
>      should also parse the bootparams to double-check that there is enough
>      low-mem space? But then if the kernel grows then 72M might not be enough -
>      you might need 82M with 3.9.

again, could with memmap= include or exclude case, so parse boot command line
is not going to handle all the case.

>
>      Perhaps a better way for this is to do:
>         1). Change 'is_enough' to check only for 4MB.
>         2). When booting as kexec, the SWIOTLB would only use 4MB instead of 64MB?

can not tell it is from kexec path. kexec just get boot loader type assigned.

>
>      Or, we could also use the post-late SWIOTLB initialization similiary to how it was
>      done on ia64. This would mean that the AMD VI code would just call the
>      .. something like this - NOT tested or even compile tested:
>
> diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
> index c1c74e0..e7fa8f7 100644
> --- a/drivers/iommu/amd_iommu.c
> +++ b/drivers/iommu/amd_iommu.c
> @@ -3173,6 +3173,24 @@ int __init amd_iommu_init_dma_ops(void)
>         if (unhandled && max_pfn > MAX_DMA32_PFN) {
>                 /* There are unhandled devices - initialize swiotlb for them */
>                 swiotlb = 1;
> +               /* Late (so no bootmem allocator) usage and only if the early SWIOTLB
> +                * hadn't been allocated (which can happen on kexec kernels booted
> +                * above 4GB). */
> +               if (!swiotlb_nr_tbl()) {
> +                       int retry = 3;
> +                       int mb_size = 64;
> +                       int rc = 0;
> +retry_me:
> +                       if (retry < 0)
> +                               panic("We tried setting %dMB for SWIOTLB but got -ENOMEM", mb_size << 1);
> +                       rc = swiotlb_late_init_with_default_size(mb_size * (1<<20));
> +                       if (rc) {
> +                               retry --;
> +                               mb_size >> 1;
> +                               goto retry_me;
> +                       }
> +                       dma_ops = &swiotlb_dma_ops;
> +               }
>         }
>
>         amd_iommu_stats_init();
>
> And then the early SWIOTLB initialization for 64MB can fail and we are still OK.

no, that is overkill.
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