Subject: Re: [PATCH v6 04/10] arm64: hyperv: Add memory alloc/free functions for Hyper-V size pages

From: Marc Zyngier <[email-masked]>
Date: Mon, 16 Mar 2020 08:30:42 +0000

On 2020-03-16 08:22, Arnd Bergmann wrote:
> On Sat, Mar 14, 2020 at 4:36 PM Michael Kelley <[email-masked]> 
> 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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