Re: [PATCH 2/6] x86: hv: hv_init.c: Replace alloc_page() with kmem_cache_alloc()

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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. 

>
>> 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)

>
>> > @@ -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