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

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Date:	Fri, 4 Dec 2015 09:20:50 +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, rjw@...ysocki.net,
	lenb@...nel.org, linux-acpi@...r.kernel.org, akataria@...are.com,
	fujita.tomonori@....ntt.co.jp, revers@...hat.com
Subject: Re: [PATCH] x86_64: enable SWIOTLB if system has SRAT memory regions
 above MAX_DMA32_PFN


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

> diff --git a/arch/x86/include/asm/acpi.h b/arch/x86/include/asm/acpi.h
> index 94c18eb..53d7951 100644
> --- a/arch/x86/include/asm/acpi.h
> +++ b/arch/x86/include/asm/acpi.h
> @@ -147,6 +147,7 @@ static inline void disable_acpi(void) { }
>  #ifdef CONFIG_ACPI_NUMA
>  extern int acpi_numa;
>  extern int x86_acpi_numa_init(void);
> +unsigned long acpi_get_max_possible_pfn(void);
>  #endif /* CONFIG_ACPI_NUMA */
>  
>  #define acpi_unlazy_tlb(x)	leave_mm(x)
> @@ -170,4 +171,8 @@ static inline pgprot_t arch_apei_get_mem_attribute(phys_addr_t addr)
>  }
>  #endif
>  
> +#ifdef CONFIG_ACPI_HOTPLUG_MEMORY
> +extern bool acpi_no_memhotplug;
> +#endif /* CONFIG_ACPI_HOTPLUG_MEMORY */
> +
>  #endif /* _ASM_X86_ACPI_H */
> diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
> index adf0392..61d5ba5 100644
> --- a/arch/x86/kernel/pci-swiotlb.c
> +++ b/arch/x86/kernel/pci-swiotlb.c
> @@ -88,7 +88,11 @@ int __init pci_swiotlb_detect_4gb(void)
>  {
>  	/* don't initialize swiotlb if iommu=off (no_iommu=1) */
>  #ifdef CONFIG_X86_64
> +#ifdef CONFIG_ACPI_NUMA
> +	if (!no_iommu && acpi_get_max_possible_pfn() > MAX_DMA32_PFN)
> +#else
>  	if (!no_iommu && max_pfn > MAX_DMA32_PFN)
> +#endif
>  		swiotlb = 1;
>  #endif
>  	return swiotlb;
> diff --git a/arch/x86/mm/srat.c b/arch/x86/mm/srat.c
> index c2aea63..21b33f0 100644
> --- a/arch/x86/mm/srat.c
> +++ b/arch/x86/mm/srat.c
> @@ -153,10 +153,20 @@ acpi_numa_processor_affinity_init(struct acpi_srat_cpu_affinity *pa)
>  
>  #ifdef CONFIG_MEMORY_HOTPLUG
>  static inline int save_add_info(void) {return 1;}
> +static unsigned long max_possible_pfn __initdata;
>  #else
>  static inline int save_add_info(void) {return 0;}
>  #endif
>  
> +unsigned long __init acpi_get_max_possible_pfn(void)
> +{
> +#ifdef CONFIG_ACPI_HOTPLUG_MEMORY
> +	if (!acpi_no_memhotplug)
> +		return max_possible_pfn;
> +#endif
> +	return max_pfn;
> +}
> +
>  /* Callback for parsing of the Proximity Domain <-> Memory Area mappings */
>  int __init
>  acpi_numa_memory_affinity_init(struct acpi_srat_mem_affinity *ma)
> @@ -203,6 +213,11 @@ 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);
>  
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +	if (max_possible_pfn < PFN_UP(end - 1))
> +		max_possible_pfn = PFN_UP(end - 1);
> +#endif
> +
>  	return 0;
>  out_err_bad_srat:
>  	bad_srat();
> diff --git a/drivers/acpi/acpi_memhotplug.c b/drivers/acpi/acpi_memhotplug.c
> index 6b0d3ef..ae38f57 100644
> --- a/drivers/acpi/acpi_memhotplug.c
> +++ b/drivers/acpi/acpi_memhotplug.c
> @@ -357,7 +357,7 @@ static void acpi_memory_device_remove(struct acpi_device *device)
>  	acpi_memory_device_free(mem_device);
>  }
>  
> -static bool __initdata acpi_no_memhotplug;
> +bool __initdata acpi_no_memhotplug;
>  
>  void __init acpi_memory_hotplug_init(void)
>  {

So I don't disagree with the fix in principle, but the implementation here is 
rather ugly - it spreads new non-obvious #ifdefs across various critical parts of 
the kernel.

For example this:

>  #ifdef CONFIG_X86_64
> +#ifdef CONFIG_ACPI_NUMA
> +	if (!no_iommu && acpi_get_max_possible_pfn() > MAX_DMA32_PFN)
> +#else
>  	if (!no_iommu && max_pfn > MAX_DMA32_PFN)
> +#endif
>  		swiotlb = 1;
>  #endif

could be cleaned up by introducing a proper max_possible_pfn variable, and setting 
it from the ACPI code - instead of exporting acpi_get_max_possible_pfn().

Another pattern is:

> +#ifdef CONFIG_ACPI_HOTPLUG_MEMORY
> +	if (!acpi_no_memhotplug)
> +		return max_possible_pfn;
> +#endif

this should be driven from the acpi_no_memhotplug knob, instead of spreading 
acpi_no_memhotplug uses to other callsites.

Furthermore, please split these various steps up into multiple steps (and first do 
the preparatory changes, then fix the bug in the end) - to make it easier to 
bisect and analyze if we regress existing functionality somewhere.

Thanks,

	Ingo
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