linux-kernel - Re: [PATCH v2 2/2] x86_64: enable SWIOTLB if system has SRAT memory regions above MAX_DMA32

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Date:	Fri, 4 Dec 2015 12:49:49 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, konrad.wilk@...cle.com, akataria@...are.com,
	fujita.tomonori@....ntt.co.jp, revers@...hat.com, riel@...hat.com,
	pbonzini@...hat.com
Subject: Re: [PATCH v2 2/2] x86_64: enable SWIOTLB if system has SRAT memory
 regions above MAX_DMA32_PFN


* Igor Mammedov <imammedo@...hat.com> wrote:

> when memory hotplug enabled system is booted with less
> than 4GB of RAM and then later more RAM is hotplugged
> 32-bit devices stop functioning with following error:
> 
>  nommu_map_single: overflow 327b4f8c0+1522 of device mask ffffffff
> 
> the reason for this is that if x86_64 system were booted
> with RAM less than 4GB, it doesn't enable SWIOTLB and
> when memory is hotplugged beyond MAX_DMA32_PFN, devices
> that expect 32-bit addresses can't handle 64-bit addresses.
> 
> Fix it by tracking max possible PFN when parsing
> memory affinity structures from SRAT ACPI table and
> enable SWIOTLB if there is hotpluggable memory
> regions beyond MAX_DMA32_PFN.
> 
> It fixes KVM guests when they use emulated devices
> (reproduces with ata_piix, e1000 and usb devices,
>  RHBZ: 1275941, 1275977, 1271527)
> It also fixes the HyperV, VMWare with emulated devices
> which are affected by this issue as well.
> 
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> ---
>  arch/x86/kernel/pci-swiotlb.c | 2 +-
>  arch/x86/mm/srat.c            | 3 +++
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
> index adf0392..7c577a1 100644
> --- a/arch/x86/kernel/pci-swiotlb.c
> +++ b/arch/x86/kernel/pci-swiotlb.c
> @@ -88,7 +88,7 @@ int __init pci_swiotlb_detect_4gb(void)
>  {
>  	/* don't initialize swiotlb if iommu=off (no_iommu=1) */
>  #ifdef CONFIG_X86_64
> -	if (!no_iommu && max_pfn > MAX_DMA32_PFN)
> +	if (!no_iommu && max_possible_pfn > MAX_DMA32_PFN)
>  		swiotlb = 1;

Ok, this series looks a lot better!

>  #endif
>  	return swiotlb;
> diff --git a/arch/x86/mm/srat.c b/arch/x86/mm/srat.c
> index c2aea63..a26bdbe 100644
> --- a/arch/x86/mm/srat.c
> +++ b/arch/x86/mm/srat.c
> @@ -203,6 +203,9 @@ acpi_numa_memory_affinity_init(struct acpi_srat_mem_affinity *ma)
>  		pr_warn("SRAT: Failed to mark hotplug range [mem %#010Lx-%#010Lx] in memblock\n",
>  			(unsigned long long)start, (unsigned long long)end - 1);
>  
> +	if (max_possible_pfn < PFN_UP(end - 1))
> +		max_possible_pfn = PFN_UP(end - 1);

Small nit, why not write this as something like:

	max_possible_pfn = max(max_possible_pfn, PFN_UP(end - 1));

?

I'd also move this second hunk to the first patch, because logically it belongs 
there (or into a third patch).

Thanks,

	Ingo
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