LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Maya Nakamura <m.maya.nakamura@gmail.com>
To: Vitaly Kuznetsov <vkuznets@redhat.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, 8 May 2019 06:46:02 +0000	[thread overview]
Message-ID: <20190508064559.GA54416@maya190131.isni1t2eisqetojrdim5hhf1se.xx.internal.cloudapp.net> (raw)
In-Reply-To: <87mukvfynk.fsf@vitty.brq.redhat.com>

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.

> >> 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.

> >> > @@ -338,7 +349,10 @@ void __init hyperv_init(void)
> >> >  	guest_id = generate_guest_id(0, LINUX_VERSION_CODE, 0);
> >> >  	wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id);
> >> >  
> >> > -	hv_hypercall_pg  = __vmalloc(PAGE_SIZE, GFP_KERNEL, PAGE_KERNEL_RX);
> >> > +	hv_hypercall_pg = kmem_cache_alloc(cachep, GFP_KERNEL);
> >> > +	if (hv_hypercall_pg)
> >> > +		set_memory_x((unsigned long)hv_hypercall_pg, 1);
> >> 
> >> _RX is not writeable, right?
> >> 
> > Yes, you are correct. I should use set_memory_ro() in addition to
> > set_memory_x().
> >
> >> > @@ -416,6 +431,7 @@ void hyperv_cleanup(void)
> >> >  	 * let hypercall operations fail safely rather than
> >> >  	 * panic the kernel for using invalid hypercall page
> >> >  	 */
> >> > +	kmem_cache_free(cachep, hv_hypercall_pg);
> >> 
> >> Please don't do that: hyperv_cleanup() is called on kexec/kdump and
> >> we're trying to do the bare minimum to allow next kernel to boot. Doing
> >> excessive work here will likely lead to consequent problems (we're
> >> already crashing the case it's kdump!).
> >> 
> > Thank you for the explanation! I will remove that.
> >
> 
> -- 
> Vitaly

  reply	other threads:[~2019-05-08  6:46 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 [this message]
2019-05-08 14:54           ` Vitaly Kuznetsov
     [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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190508064559.GA54416@maya190131.isni1t2eisqetojrdim5hhf1se.xx.internal.cloudapp.net \
    --to=m.maya.nakamura@gmail.com \
    --cc=haiyangz@microsoft.com \
    --cc=kys@microsoft.com \
    --cc=linux-hyperv@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mikelley@microsoft.com \
    --cc=sashal@kernel.org \
    --cc=sthemmin@microsoft.com \
    --cc=vkuznets@redhat.com \
    --cc=x86@kernel.org \
    --subject='Re: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache_alloc()' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).