Re: [PATCH v4 2/3] system/memory: Use qemu_ram_move() for directly accessible regions

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 27, 2026 at 01:51:45PM +0100, Peter Maydell wrote:
> I think we could usefully expand this comment, because the reasons
> we need it are not immediately obvious. How about:
> 
> ===begin===
> Move @n bytes from @src to @dst; the memory areas may overlap.
> This provides the same semantics as memmove(), plus an additional
> stronger guarantee: if @n is 1, 2 or 4 or 8 bytes, and @src
> and @dst are both naturally aligned for that access size, and
> the memory areas do not overlap, then both the load and the store
> will be done as a single atomic access (with the semantics of
> qatomic_read() and qatomic_set()).
> 
> This is the underlying function that we use to implement accesses
> by a guest vCPU or a device DMA operation to a ram block. The
> atomic guarantee is needed for two major cases:
>  - when the ram block is backed by a PCI BAR passed through
>    from a host device (and so it might be hardware registers
>    that must be accessed exactly once at the right width)
>  - when an emulated device updates a data structure shared in
>    guest memory with guest software (e.g. a network device's
>    set of tx and rx descriptor blocks), if a write to memory
>    is accidentally performed multiple times then it can break
>    the guest code.

Maybe also append it with "the guest code when it busy polls the guest
memory"; I just found that the polling model isn't something obvious too
when reading it first.

> 
> We don't attempt to perform the exact access when it would
> be unaligned, because this can't necessarily be done on
> all host architectures; although this is strictly speaking
> not doing what would happen on real hardware, we don't think
> there are going to be situations where that matters in practice.

Oh yes, mentioning the unaligned part of discussion would also be nice, I
forgot it when replying.

Thanks,

> ===endit===

-- 
Peter Xu