Re: [Xen-devel] [PATCH 2/2] x86/hvm/emulate: make sure rep I/O emulation does not cross GFN boundaries

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/08/18 11:37, Paul Durrant wrote:
> When emulating a rep I/O operation it is possible that the ioreq will
> describe a single operation that spans multiple GFNs. This is fine as long
> as all those GFNs fall within an MMIO region covered by a single device
> model, but unfortunately the higher levels of the emulation code do not
> guarantee that. This is something that should almost certainly be fixed,
> but in the meantime this patch makes sure that MMIO is truncated at GFN
> boundaries and hence the appropriate device model is re-evaluated for each
> target GFN.
>
> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> ---
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
>  xen/arch/x86/hvm/emulate.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
> index 8385c62145..d6a81ec4d1 100644
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -184,8 +184,23 @@ static int hvmemul_do_io(
>          hvmtrace_io_assist(&p);
>      }
>  
> -    vio->io_req = p;
> +    /*
> +     * Make sure that we truncate rep MMIO at any GFN boundary. This is
> +     * necessary to ensure that the correct device model is targetted
> +     * or that we correctly handle a rep op spanning MMIO and RAM.
> +     */
> +    if ( unlikely(p.count > 1) && p.type == IOREQ_TYPE_COPY )
> +    {
> +        unsigned long off = p.addr & ~PAGE_MASK;
>  
> +        p.count = min_t(unsigned long,
> +                        p.count,
> +                        p.df ?
> +                        (off + p.size) / p.size :
> +                        (PAGE_SIZE - off) / p.size);

(p.df ? (off + p.size) : (PAGE_SIZE - off)) / p.size

?

> +    }
> +
> +    vio->io_req = p;

You'll get a cleaner patch if you retain the newline between these two.

Both can be fixed up on commit.  From a functional point of view, this
looks fine.

~Andrew

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