Re: [Qemu-devel] [PATCH v2 1/4] ahci: Do not ignore memory access read size

[Eric Blake]

On 06/16/2015 10:02 AM, John Snow wrote:
> The only guidance the AHCI specification gives on memory access is:
> "Register accesses shall have a maximum size of 64-bits; 64-bit access
> must not cross an 8-byte alignment boundary."
> 
> I interpret this to mean that aligned or unaligned 1, 2 and 4 byte
> accesses should work, as well as aligned 8 byte accesses.
> 
> In practice, a real Q35/ICH9 responds to 1, 2, 4 and 8 byte reads
> regardless of alignment. Windows 7 can be observed making 1 byte
> reads to the middle of 32 bit registers to fetch error codes.
> 
> Introduce a wrapper to support unaligned accesses to AHCI.
> This wrapper will support aligned 8 byte reads, but will make
> no effort to support unaligned 8 byte reads, which although they
> will work on real hardware, are not guaranteed to work and do
> not appear to be used by either Windows or Linux.
> 
> Signed-off-by: John Snow <address@hidden>
> ---
>  hw/ide/ahci.c | 27 +++++++++++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)
> 

> +/**
> + * AHCI 1.3 section 3 ("HBA Memory Registers")
> + * Support unaligned 8/16/32 bit reads, and 64 bit aligned reads.
> + * Caller is responsible for masking unwanted higher order bytes.
> + */
> +static uint64_t ahci_mem_read(void *opaque, hwaddr addr, unsigned size)
> +{
> +    hwaddr aligned = addr & ~0x3;

This actually supports 4-byte aligned 8-byte reads (which is an
unaligned 8-byte read).  Doesn't matter; no guest should be relying on it.

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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