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    SubjectRe: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache_alloc()
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    Maya Nakamura <m.maya.nakamura@gmail.com> writes:

    > On Fri, Apr 12, 2019 at 09:52:47AM +0200, Vitaly Kuznetsov wrote:
    >> Maya Nakamura <m.maya.nakamura@gmail.com> writes:
    >>
    >> > On Fri, Apr 05, 2019 at 01:31:02PM +0200, Vitaly Kuznetsov wrote:
    >> >> Maya Nakamura <m.maya.nakamura@gmail.com> writes:
    >> >>
    >> >> > @@ -98,18 +99,20 @@ EXPORT_SYMBOL_GPL(hyperv_pcpu_input_arg);
    >> >> > u32 hv_max_vp_index;
    >> >> > EXPORT_SYMBOL_GPL(hv_max_vp_index);
    >> >> >
    >> >> > +struct kmem_cache *cachep;
    >> >> > +EXPORT_SYMBOL_GPL(cachep);
    >> >> > +
    >> >> > static int hv_cpu_init(unsigned int cpu)
    >> >> > {
    >> >> > u64 msr_vp_index;
    >> >> > struct hv_vp_assist_page **hvp = &hv_vp_assist_page[smp_processor_id()];
    >> >> > void **input_arg;
    >> >> > - struct page *pg;
    >> >> >
    >> >> > input_arg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
    >> >> > - pg = alloc_page(GFP_KERNEL);
    >> >> > - if (unlikely(!pg))
    >> >> > + *input_arg = kmem_cache_alloc(cachep, GFP_KERNEL);
    >> >>
    >> >> I'm not sure use of kmem_cache is justified here: pages we allocate are
    >> >> not cache-line and all these allocations are supposed to persist for the
    >> >> lifetime of the guest. In case you think that even on x86 it will be
    >> >> possible to see PAGE_SIZE != HV_HYP_PAGE_SIZE you can use alloc_pages()
    >> >> instead.
    >> >>
    >> > Thank you for your feedback, Vitaly!
    >> >
    >> > Will you please tell me how cache-line relates to kmem_cache?
    >> >
    >> > I understand that alloc_pages() would work when PAGE_SIZE <=
    >> > HV_HYP_PAGE_SIZE, but I think that it would not work if PAGE_SIZE >
    >> > HV_HYP_PAGE_SIZE.
    >>
    >> Sorry, my bad: I meant to say "not cache-like" (these allocations are
    >> not 'cache') but the typo made it completely incomprehensible.
    >
    > No worries! Thank you for sharing your thoughts with me, Vitaly.
    >
    > Do you know of any alternatives to kmem_cache that can allocate memory
    > in a specified size (different than a guest page size) with alignment?
    > Memory allocated by alloc_page() is aligned but limited to the guest
    > page size, and kmalloc() can allocate a particular size but it seems
    > that it does not guarantee alignment. I am asking this while considering
    > the changes for architecture independent code.
    >

    I think we can consider these allocations being DMA-like (because
    Hypervisor accesses this memory too) so you can probably take a look at
    dma_pool_create()/dma_pool_alloc() and friends.

    >> >> Also, in case the idea is to generalize stuff, what will happen if
    >> >> PAGE_SIZE > HV_HYP_PAGE_SIZE? Who will guarantee proper alignment?
    >> >>
    >> >> I think we can leave hypercall arguments, vp_assist and similar pages
    >> >> alone for now: the code is not going to be shared among architectures
    >> >> anyways.
    >> >>
    >> > About the alignment, kmem_cache_create() aligns memory with its third
    >> > parameter, offset.
    >>
    >> Yes, I know, I was trying to think about a (hypothetical) situation when
    >> page sizes differ: what would be the memory alignment requirements from
    >> the hypervisor for e.g. hypercall arguments? In case it's always
    >> HV_HYP_PAGE_SIZE we're good but could it be PAGE_SIZE (for e.g. TLB
    >> flush hypercall)? I don't know. For x86 this discussion probably makes
    >> no sense. I'm, however, struggling to understand what benefit we will
    >> get from the change. Maybe just leave it as-is for now and fix
    >> arch-independent code only? And later, if we decide to generalize this
    >> code, make another approach? (Not insisting, just a suggestion)
    >
    > Thank you for the suggestion, Vitaly!
    >
    > The introduction of HV_HYP_PAGE_SIZE is weighing the assumption of the
    > future page size—it can be bigger based on the general trend, not
    > smaller, which is a reasonable assumption, I think.
    >

    Let's spell it out (as BUILD_BUG_ON(HV_HYP_PAGE_SIZE < PAGE_SIZE) or
    something like that) then to make it clear.

    --
    Vitaly

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