Re: [Xen-devel] [PATCH v4 2/7] x86/hyperv: setup hypercall page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.01.2020 22:31, Andrew Cooper wrote:
> On 22/01/2020 20:23, Wei Liu wrote:
>> --- a/xen/arch/x86/e820.c
>> +++ b/xen/arch/x86/e820.c
>> +     */
>> +        unsigned int phys_bits = find_phys_addr_bits();
>> +        unsigned long hcall_pfn =
>> +          ((1ull << phys_bits) - 1) >> PAGE_SHIFT;
> 
> (1ull << (phys_bits - PAGE_SHIFT)) - 1 is equivalent, and doesn't
> require a right shift.  I don't know if the compiler is smart enough to
> make this optimisation automatically.

It's not allowed to, without having a way to know that phys_bits is
no less than PAGE_SHIFT.

Jan

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