Re: [PATCH 1/2] memattrs: Get rid of bit fields

[Paolo Bonzini <pbonzini@redhat.com>]

On 1/20/25 17:38, Zhao Liu wrote:
> Thanks for the reminder, yes it is currently full. I found I missed
> a commnet from Paolo [*], that he suggested only convert `unspecified`
> to a bool. My bad :-(
> 
> It still raises the size to 8 bytes but saves spare space, like:
> 
> typedef struct MemTxAttrs {
>      unsigned int secure:1;
>      unsigned int space:2;
>      unsigned int user:1;
>      unsigned int memory:1;
>      unsigned int requester_id:16;
>      unsigned int pid:8;
>      bool unspecified;
>      uint8_t _reserved1;
>      uint16_t _reserved2;
> } MemTxAttrs;
> 
> Similar to your comment above, to get pakced structure, I think I need
> push `unspecified` field down to other bit fields.

Right, this would allow for a 16-bit PASID, 19 free bits and no QEMU_PACKED:

typedef struct MemTxAttrs {
     bool unspecified;
     uint8_t int secure:1;
     uint8_t int space:2;
     uint8_t int user:1;
     uint8_t memory:1;
     uint16_t requester_id;
     uint16_t pid;
     uint16_t _reserved;
} MemTxAttrs;
QEMU_BUILD_BUG_ON(sizeof(MemTxAttrs) > 8);

Together with const_zero!() that would be fine for both C and Rust, and 
a bit more efficient too.

Paolo