Re: [PATCH v1 02/16] irqchip: gicv3-its: Add helpers for handling 52bit address

[Suzuki K Poulose <Suzuki.Poulose@arm.com>]

On 07/02/18 15:10, Christoffer Dall wrote:
> Hi Suzuki,
> 
> On Tue, Jan 09, 2018 at 07:03:57PM +0000, Suzuki K Poulose wrote:
>> Add helpers for encoding/decoding 52bit address in GICv3 ITS BASER
>> register. When ITS uses 64K page size, the 52bits of physical address
>> are encoded in BASER[47:12] as follows :
>>
>>   Bits[47:16] of the register => bits[47:16] of the physical address
>>   Bits[15:12] of the register => bits[51:48] of the physical address
>>                                  bits[15:0] of the physical address are 0.
>>
>> Also adds a mask for CBASER address. This will be used for adding 52bit
>> support for VGIC ITS. More importantly ignore the upper bits if 52bit
>> support is not enabled.
>>
>> Cc: Shanker Donthineni <shankerd@codeaurora.org>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> ---


>> +
>> +/*
>> + * With 64K page size, the physical address can be upto 52bit and
>> + * uses the following encoding in the GITS_BASER[47:12]:
>> + *
>> + * Bits[47:16] of the register => bits[47:16] of the base physical address.
>> + * Bits[15:12] of the register => bits[51:48] of the base physical address.
>> + *                                bits[15:0] of the base physical address are 0.
>> + * Clear the upper bits if the kernel doesn't support 52bits.
>> + */
>> +#define GITS_BASER_ADDR64K_LO_MASK	GENMASK_ULL(47, 16)
>> +#define GITS_BASER_ADDR64K_HI_SHIFT	12
>> +#define GITS_BASER_ADDR64K_HI_MOVE	(48 - GITS_BASER_ADDR64K_HI_SHIFT)
>> +#define GITS_BASER_ADDR64K_HI_MASK	(GITS_PA_HI_MASK << GITS_BASER_ADDR64K_HI_SHIFT)
>> +#define GITS_BASER_ADDR64K_TO_PHYS(x)					\
>> +	(((x) & GITS_BASER_ADDR64K_LO_MASK) | 				\
>> +	 (((x) & GITS_BASER_ADDR64K_HI_MASK) << GITS_BASER_ADDR64K_HI_MOVE))
>> +#define GITS_BASER_ADDR64K_FROM_PHYS(p)					\
>> +	(((p) & GITS_BASER_ADDR64K_LO_MASK) | 				\
>> +	 (((p) >> GITS_BASER_ADDR64K_HI_MOVE) & GITS_BASER_ADDR64K_HI_MASK))
> 
> I don't understand why you need this masking logic embedded in these
> macros?  Isn't it strictly an error if anyone passes a physical address
> with any of bits [51:48] set to the ITS on a system that doesn't support
> 52 bit PAs, and just silently masking off those bits could lead to some
> interesting cases.

What do you think is the best way to handle such cases ? May be I could add
some checks where we get those addresses and handle it before we use this
macro ?

> 
> This is also notably more difficult to read than the existing macro.
> 
> If anything, I think it would be more useful to have
> GITS_BASER_TO_PHYS(x) and GITS_PHYS_TO_BASER(x) which takes into account
> CONFIG_ARM64_64K_PAGES.

I thought the 64K_PAGES is not kernel page size, but the page-size configured
by the "requester" for ITS. So, it doesn't really mean CONFIG_ARM64_64K_PAGES.
But the other way around, we can't handle 52bit address unless CONFIG_ARM64_64K_PAGES
is selected. Also, if the guest uses a 4K page size and uses a 48 bit address,
we could potentially mask Bits[15:12] to 0, which is not nice.

So I still think we need to have a special macro for handling addresses with 64K
page size in ITS.

Thanks
Suzuki