linux-kernel - Re: [RFC PATCH v2 08/32] x86: Use PAGE_KERNEL protection for ioremap of memory page

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Date:   Thu, 16 Mar 2017 15:04:42 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...e.de>, Brijesh Singh <brijesh.singh@....com>
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Subject: Re: [RFC PATCH v2 08/32] x86: Use PAGE_KERNEL protection for ioremap
 of memory page

On 3/7/2017 8:59 AM, Borislav Petkov wrote:
> On Thu, Mar 02, 2017 at 10:13:32AM -0500, Brijesh Singh wrote:
>> From: Tom Lendacky <thomas.lendacky@....com>
>>
>> In order for memory pages to be properly mapped when SEV is active, we
>> need to use the PAGE_KERNEL protection attribute as the base protection.
>> This will insure that memory mapping of, e.g. ACPI tables, receives the
>> proper mapping attributes.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
>> ---
>
>> diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
>> index c400ab5..481c999 100644
>> --- a/arch/x86/mm/ioremap.c
>> +++ b/arch/x86/mm/ioremap.c
>> @@ -151,7 +151,15 @@ static void __iomem *__ioremap_caller(resource_size_t phys_addr,
>>                 pcm = new_pcm;
>>         }
>>
>> +       /*
>> +        * If the page being mapped is in memory and SEV is active then
>> +        * make sure the memory encryption attribute is enabled in the
>> +        * resulting mapping.
>> +        */
>>         prot = PAGE_KERNEL_IO;
>> +       if (sev_active() && page_is_mem(pfn))
>
> Hmm, a resource tree walk per ioremap call. This could get expensive for
> ioremap-heavy workloads.
>
> __ioremap_caller() gets called here during boot 55 times so not a whole
> lot but I wouldn't be surprised if there were some nasty use cases which
> ioremap a lot.
>
> ...
>
>> diff --git a/kernel/resource.c b/kernel/resource.c
>> index 9b5f044..db56ba3 100644
>> --- a/kernel/resource.c
>> +++ b/kernel/resource.c
>> @@ -518,6 +518,46 @@ int __weak page_is_ram(unsigned long pfn)
>>  }
>>  EXPORT_SYMBOL_GPL(page_is_ram);
>>
>> +/*
>> + * This function returns true if the target memory is marked as
>> + * IORESOURCE_MEM and IORESOUCE_BUSY and described as other than
>> + * IORES_DESC_NONE (e.g. IORES_DESC_ACPI_TABLES).
>> + */
>> +static int walk_mem_range(unsigned long start_pfn, unsigned long nr_pages)
>> +{
>> +	struct resource res;
>> +	unsigned long pfn, end_pfn;
>> +	u64 orig_end;
>> +	int ret = -1;
>> +
>> +	res.start = (u64) start_pfn << PAGE_SHIFT;
>> +	res.end = ((u64)(start_pfn + nr_pages) << PAGE_SHIFT) - 1;
>> +	res.flags = IORESOURCE_MEM | IORESOURCE_BUSY;
>> +	orig_end = res.end;
>> +	while ((res.start < res.end) &&
>> +		(find_next_iomem_res(&res, IORES_DESC_NONE, true) >= 0)) {
>> +		pfn = (res.start + PAGE_SIZE - 1) >> PAGE_SHIFT;
>> +		end_pfn = (res.end + 1) >> PAGE_SHIFT;
>> +		if (end_pfn > pfn)
>> +			ret = (res.desc != IORES_DESC_NONE) ? 1 : 0;
>> +		if (ret)
>> +			break;
>> +		res.start = res.end + 1;
>> +		res.end = orig_end;
>> +	}
>> +	return ret;
>> +}
>
> So the relevant difference between this one and walk_system_ram_range()
> is this:
>
> -			ret = (*func)(pfn, end_pfn - pfn, arg);
> +			ret = (res.desc != IORES_DESC_NONE) ? 1 : 0;
>
> so it seems to me you can have your own *func() pointer which does that
> IORES_DESC_NONE comparison. And then you can define your own workhorse
> __walk_memory_range() which gets called by both walk_mem_range() and
> walk_system_ram_range() instead of almost duplicating them.
>
> And looking at walk_system_ram_res(), that one looks similar too except
> the pfn computation. But AFAICT the pfn/end_pfn things are computed from
> res.start and res.end so it looks to me like all those three functions
> are crying for unification...

I'll take a look at what it takes to consolidate these with a pre-patch. 
Then I'll add the new support.

Thanks,
Tom

>