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Re: [Qemu-block] [Qemu-devel] [PATCH-for-4.1 v4 2/5] hw/block/pflash_cfi

From: Peter Maydell
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH-for-4.1 v4 2/5] hw/block/pflash_cfi01: Use the correct READ_ARRAY value
Date: Fri, 12 Jul 2019 16:15:06 +0100

On Thu, 11 Jul 2019 at 16:58, Philippe Mathieu-Daudé <address@hidden> wrote:
> In the "Read Array Flowchart" the command has a value of 0xFF.
> In the document [*] the "Read Array Flowchart", the READ_ARRAY
> command has a value of 0xff.
> Use the correct value in the pflash model.
> There is no change of behavior in the guest, because:
> - when the guest were sending 0xFF, the reset_flash label
>   was setting the command value as 0x00
> - 0x00 was used internally for READ_ARRAY
> To keep migration with older versions behaving correctly, we
> decide to always migrate the READ_ARRAY as 0x00.
> [*] "Common Flash Interface (CFI) and Command Sets"
>     (Intel Application Note 646)
>     Appendix B "Basic Command Set"
> Reviewed-by: John Snow <address@hidden>
> Reviewed-by: Alistair Francis <address@hidden>
> Regression-tested-by: Laszlo Ersek <address@hidden>
> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> ---

These changes look correct as far as they go, but are
we sure that command value 0x00 will never be a valid
command in some future version? If it ever does, then we
have a problem because we can't distinguish "0xff with
a silly encoding" from "really 0x00" in the incoming
migration data stream.

If we're 100% confident that there will never be a true
command 0x00 then this approach is OK.

-- PMM

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