Re: [Xen-devel] [PATCH] x86/HVM: fix PM timer I/O port range version check

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.01.2020 14:56, Roger Pau Monné wrote:
> On Tue, Jan 28, 2020 at 02:17:51PM +0100, Jan Beulich wrote:
>> XOR-ing two arbitrary non-equal values may produce 1 even if both values
>> are different from both 0 and 1 (2 and 3 would fit, for example). Use OR
>> instead, which together with the earlier bailing upon finding
>> "version == old_version" achieves the intended effect.
>>
>> Fixes: f0ad21c499f7 ("x86 hvm: Introduce pmtimer_change_ioport and 
>> HVM_PARAM_ACPI_IOPORTS_LOCATION")
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, but I withdraw this patch. The code is correct as is, just
a little less obviously correct than with the use of |. As long as
there's no way for the value stored in Xen to be larger than 1,
there's also no way for a larger value to pass the check in
question.

Instead I notice the upper 32 bits of the value are silently
ignored. I'll submit a new patch addressing that instead.

Jan

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