From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Maya Nakamura <m.maya.nakamura@gmail.com>
Cc: mikelley@microsoft.com, kys@microsoft.com,
haiyangz@microsoft.com, sthemmin@microsoft.com,
sashal@kernel.org, x86@kernel.org, linux-hyperv@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache_alloc()
Date: Wed, 08 May 2019 10:54:59 -0400 [thread overview]
Message-ID: <87mujxro70.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20190508064559.GA54416@maya190131.isni1t2eisqetojrdim5hhf1se.xx.internal.cloudapp.net>
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
next prev parent reply other threads:[~2019-05-08 14:55 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-05 1:11 [PATCH 0/6] hv: Remove dependencies on guest page size Maya Nakamura
2019-04-05 1:13 ` [PATCH 1/6] x86: hv: hyperv-tlfs.h: Create and use Hyper-V page definitions Maya Nakamura
2019-04-05 1:14 ` [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache_alloc() Maya Nakamura
2019-04-05 11:31 ` Vitaly Kuznetsov
2019-04-12 7:24 ` Maya Nakamura
2019-04-12 7:52 ` Vitaly Kuznetsov
2019-05-08 6:46 ` Maya Nakamura
2019-05-08 14:54 ` Vitaly Kuznetsov [this message]
[not found] ` <BYAPR21MB1317AC7CA4B242106FCAD698CC320@BYAPR21MB1317.namprd21.prod.outlook.com>
2019-05-08 19:53 ` Vitaly Kuznetsov
[not found] ` <MN2PR21MB1232C6ABA5DAC847C8A910E1D70C0@MN2PR21MB1232.namprd21.prod.outlook.com>
2019-05-10 13:21 ` Vitaly Kuznetsov
[not found] ` <BYAPR21MB1221962ED2DD7FEE19E7DAB6D70C0@BYAPR21MB1221.namprd21.prod.outlook.com>
2019-05-10 17:45 ` Vitaly Kuznetsov
2019-04-05 1:16 ` [PATCH 3/6] hv: vmbus: Replace page definition with Hyper-V specific one Maya Nakamura
2019-04-05 1:17 ` [PATCH 4/6] x86: hv: mmu.c: Replace page definitions with Hyper-V specific ones Maya Nakamura
2019-04-05 11:10 ` Vitaly Kuznetsov
[not found] ` <DM5PR2101MB091843B6DD7A11C2C27917F1D7280@DM5PR2101MB0918.namprd21.prod.outlook.com>
2019-04-12 6:58 ` Vitaly Kuznetsov
2019-04-05 1:19 ` [PATCH 5/6] HID: hv: Remove dependencies on PAGE_SIZE for ring buffer Maya Nakamura
2019-04-05 1:20 ` [PATCH 6/6] Input: " Maya Nakamura
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