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SubjectRe: [PATCH v6 04/10] arm64: hyperv: Add memory alloc/free functions for Hyper-V size pages
On 2020-03-16 08:22, Arnd Bergmann wrote:
> On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <mikelley@microsoft.com>
> wrote:
>> /*
>> + * Functions for allocating and freeing memory with size and
>> + * alignment HV_HYP_PAGE_SIZE. These functions are needed because
>> + * the guest page size may not be the same as the Hyper-V page
>> + * size. We depend upon kmalloc() aligning power-of-two size
>> + * allocations to the allocation size boundary, so that the
>> + * allocated memory appears to Hyper-V as a page of the size
>> + * it expects.
>> + *
>> + * These functions are used by arm64 specific code as well as
>> + * arch independent Hyper-V drivers.
>> + */
>> +
>> +void *hv_alloc_hyperv_page(void)
>> +{
>> + BUILD_BUG_ON(PAGE_SIZE < HV_HYP_PAGE_SIZE);
>> + return kmalloc(HV_HYP_PAGE_SIZE, GFP_KERNEL);
>> +}
>> +EXPORT_SYMBOL_GPL(hv_alloc_hyperv_page);
>
> I don't think there is any guarantee that kmalloc() returns
> page-aligned
> allocations in general.

I believe that guarantee came with 59bb47985c1db ("mm, sl[aou]b:
guarantee
natural alignment for kmalloc(power-of-two)").

> How about using get_free_pages() to implement this?

This would certainly work, at the expense of a lot of wasted memory when
PAGE_SIZE isn't 4k.

Thanks,

M.
--
Jazz is not dead. It just smells funny...

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