Re: [Qemu-devel] [RFC v4 1/9] exec.c: Add new exclusive bitmap to ram_list

[Paolo Bonzini]

On 07/08/2015 19:03, Alvise Rigo wrote:
> +static inline int cpu_physical_memory_excl_atleast_one_clean(ram_addr_t addr)
> +{
> +    unsigned long *bitmap = ram_list.dirty_memory[DIRTY_MEMORY_EXCLUSIVE];
> +    unsigned long next, end;
> +
> +    if (likely(smp_cpus <= BITS_PER_LONG)) {

This only works if smp_cpus divides BITS_PER_LONG, i.e. BITS_PER_LONG %
smp_cpus == 0.

> +        unsigned long mask = (1 << smp_cpus) - 1;
> +
> +        return
> +            (mask & (bitmap[BIT_WORD(EXCL_BITMAP_GET_OFFSET(addr))] >>
> +            (EXCL_BITMAP_GET_OFFSET(addr) & (BITS_PER_LONG-1)))) != mask;
> +    }
> +
> +    end = BIT_WORD(EXCL_BITMAP_GET_OFFSET(addr)) + smp_cpus;
> +    next = find_next_zero_bit(bitmap, end,
> +                              BIT_WORD(EXCL_BITMAP_GET_OFFSET(addr)));
> +
> +    return next < end;

> +static inline int cpu_physical_memory_excl_is_dirty(ram_addr_t addr,
> +                                                    unsigned long cpu)
> +{
> +    unsigned long *bitmap = ram_list.dirty_memory[DIRTY_MEMORY_EXCLUSIVE];
> +    unsigned long end, next;
> +    uint32_t add;
> +
> +    assert(cpu <= smp_cpus);
> +
> +    if (likely(smp_cpus <= BITS_PER_LONG)) {
> +        cpu = (cpu == smp_cpus) ? (1 << cpu) - 1 : (1 << cpu);
> +
> +        return cpu & (bitmap[BIT_WORD(EXCL_BITMAP_GET_OFFSET(addr))] >>
> +                     (EXCL_BITMAP_GET_OFFSET(addr) & (BITS_PER_LONG-1)));
> +    }
> +
> +    add = (cpu == smp_cpus) ? 0 : 1;

Why not have a separate function for the cpu == smp_cpus case?

I don't think real hardware has ll/sc per CPU.  Can we have the bitmap as:

- 0 if one or more CPUs have the address set to exclusive, _and_ no CPU
has done a concurrent access

- 1 if no CPUs have the address set to exclusive, _or_ one CPU has done
a concurrent access.

Then:

- ll sets the bit to 0, and requests a flush if it was 1

- when setting a TLB entry, set it to TLB_EXCL if the bitmap has 0

- in the TLB_EXCL slow path, set the bit to 1 and, for conditional
stores, succeed if the bit was 0

- when removing an exclusive entry, set the bit to 1

Paolo