Re: [RFC PATCH V2] xen/gnttab: Store frame GFN in struct page_info on Arm

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.09.2021 00:13, Oleksandr wrote:
> On 15.09.21 13:06, Jan Beulich wrote:
>> On 14.09.2021 22:44, Oleksandr Tyshchenko wrote:
>>> --- a/xen/include/asm-arm/mm.h
>>> +++ b/xen/include/asm-arm/mm.h
>>> @@ -98,9 +98,18 @@ struct page_info
>>>   #define PGT_writable_page PG_mask(1, 1)  /* has writable mappings?        
>>>  */
>>>   #define PGT_type_mask     PG_mask(1, 1)  /* Bits 31 or 63.                
>>>  */
>>>   
>>> - /* Count of uses of this frame as its current type. */
>>> -#define PGT_count_width   PG_shift(2)
>>> -#define PGT_count_mask    ((1UL<<PGT_count_width)-1)
>>> + /* 3-bit count of uses of this frame as its current type. */
>>> +#define PGT_count_base    PG_shift(4)
>>> +#define PGT_count_mask    PG_mask(7, 4)
>>> +
>>> +/*
>>> + * Stored in bits [27:0] or [59:0] GFN if page is used for grant table 
>>> frame.
>> I don't know enough Arm details to tell whether this is properly
>> one bit more than the maximum number of physical address bits.
>> Without the extra bit you wouldn't be able to tell apart a
>> guest specified GFN matching the value of PGT_INVALID_FRAME_GFN
>> from an entry which was set from INVALID_GFN.
> Really good point.
> 
> 1. On Arm64 the p2m_ipa_bits could (theoretically) be 64-bit which, I 
> assume, corresponds to the maximum guest physical address (1 << 64) - 1 
> = 0xFFFFFFFFFFFFFFFF.
> To store that GFN we need 52-bit. But, we provide 60-bit field which is 
> more than enough, I think. Practically, the maximum supported 
> p2m_ipa_bits is 48-bit, so the maximum supported GFN will occupy 36-bit 
> only. Everything is ok here.
> 2. On Arm32 the p2m_ipa_bits is 40-bit which, I assume, corresponds to 
> the maximum guest physical address (1 << 40) - 1 = 0xFFFFFFFFFF. To 
> store that GFN we need 28-bit. If I did the calculation correctly, what 
> we have on Arm32 is that PGT_INVALID_FRAME_GFN == maximum guest physical 
> address and it looks like we need and extra bit on Arm32. Do you think 
> we need to borrow one more bit from the count portion to stay on the 
> safe side?

I think so, unless there are further restrictions on the GFN range
that I'm unaware of.

For 64-bit, if you only need 52 bits, why do you make the field 60
bits wide? I'd recommend against "wasting" bits. Better keep the
count field as wide as possible.

>>> + * This only valid for the xenheap pages.
>>> + */
>>> +#define PGT_gfn_width     PG_shift(4)
>>> +#define PGT_gfn_mask      ((1UL<<PGT_gfn_width)-1)
>> Any reason you don't use PG_mask() here? Any open-coding is prone
>> to people later making mistakes.
> I failed to come up with idea how to do that without #ifdef. As GFN 
> starts at bit 0 different first parameter would be needed for PG_mask on 
> 32-bit and 64-bit systems.
> I wonder whether PGC_count_mask/PGT_count_mask are open-coded on Arm/x86 
> because of the same reason.

Hmm, that pre-existing open-coding isn't nice, but is perhaps a
good enough reason to keep what you have. (Personally I wouldn't
be afraid of adding an #ifdef here, but I realize views there may
differ.)

Jan