Re: [PATCH v3] xen/riscv: Increase XEN_VIRT_SIZE

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

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On 4/10/25 10:48 AM, Jan Beulich wrote:
> On 09.04.2025 21:01, Oleksii Kurochko wrote:
>> --- a/xen/arch/riscv/include/asm/mm.h
>> +++ b/xen/arch/riscv/include/asm/mm.h
>> @@ -9,6 +9,7 @@
>>   #include <xen/mm-frame.h>
>>   #include <xen/pdx.h>
>>   #include <xen/pfn.h>
>> +#include <xen/sections.h>
>>   #include <xen/types.h>
>>   
>>   #include <asm/page-bits.h>
>> @@ -35,6 +36,11 @@ static inline void *maddr_to_virt(paddr_t ma)
>>       return (void *)va;
>>   }
>>   
>> +#define is_init_section(p) ({                   \
>> +    char *p_ = (char *)(unsigned long)(p);      \
>> +    (p_ >= __init_begin) && (p_ < __init_end);  \
>> +})
> I think this wants to be put in xen/sections.h, next to where __init_{begin,end}
> are declared. But first it wants making const-correct, to eliminate the potential
> of it indirectly casting away const-ness from the incoming argument.
>
> (At some point related stuff wants moving from kernel.h to sections.h, I suppose.
> And at that point they will all want to have const added.)

Sure, I'll change to 'const char *p_ = (const char*)(unsigned long)(p)'.

>> --- a/xen/arch/riscv/mm.c
>> +++ b/xen/arch/riscv/mm.c
>> @@ -31,20 +31,24 @@ unsigned long __ro_after_init phys_offset; /* = load_start - XEN_VIRT_START */
>>   #define LOAD_TO_LINK(addr) ((unsigned long)(addr) - phys_offset)
>>   
>>   /*
>> - * It is expected that Xen won't be more then 2 MB.
>> + * It is expected that Xen won't be more then XEN_VIRT_SIZE MB.
> Why "MB" when the macro already expands to MB(16)?

It should be really dropped, no need for MB in the comment.

>
>>    * The check in xen.lds.S guarantees that.
>> - * At least 3 page tables (in case of Sv39 ) are needed to cover 2 MB.
>> - * One for each page level table with PAGE_SIZE = 4 Kb.
>>    *
>> - * One L0 page table can cover 2 MB(512 entries of one page table * PAGE_SIZE).
>> + * Root page table is shared with the initial mapping and is declared
>> + * separetely. (look at stage1_pgtbl_root)
> Nit: separately
>
>>    *
>> - * It might be needed one more page table in case when Xen load address
>> - * isn't 2 MB aligned.
>> + * An amount of page tables between root page table and L0 page table
>> + * (in the case of Sv39 it covers L1 table):
>> + *   (CONFIG_PAGING_LEVELS - 2) are needed for an identity mapping and
>> + *   the same amount are needed for Xen.
>>    *
>> - * CONFIG_PAGING_LEVELS page tables are needed for the identity mapping,
>> - * except that the root page table is shared with the initial mapping
>> + * An amount of L0 page tables:
>> + *   (512 entries of one L0 page table covers 2MB == 1<<XEN_PT_LEVEL_SHIFT(1))
>> + *   XEN_VIRT_SIZE >> XEN_PT_LEVEL_SHIFT(1) are needed for Xen and
>> + *   one L0 is needed for indenity mapping.
> Nit: identity
>
> But more importantly, where's this one L0 ...
>
>>    */
>> -#define PGTBL_INITIAL_COUNT ((CONFIG_PAGING_LEVELS - 1) * 2 + 1)
>> +#define PGTBL_INITIAL_COUNT ((CONFIG_PAGING_LEVELS - 2) * 2 + \
>> +                             (XEN_VIRT_SIZE >> XEN_PT_LEVEL_SHIFT(1)))
> .... in this calculation?

L0 for identity mapping is really missed.

Thanks.

~ Oleksii

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