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Re: [PATCH] x86, kvm: Handle PFNs outside of kernel reach when touching GPTEs

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  Re: [PATCH] x86, kvm: Handle PFNs outside of kernel reach when  touching GPTEs "Sironi, Filippo" <sironi@amazon.de> - 2017-04-12 15:20 +0200
    Re: [PATCH] x86, kvm: Handle PFNs outside of kernel reach when  touching GPTEs Xiao Guangrong <guangrong.xiao@gmail.com> - 2017-04-17 13:30 +0200
      Re: [PATCH] x86, kvm: Handle PFNs outside of kernel reach when  touching GPTEs Paolo Bonzini <pbonzini@redhat.com> - 2017-04-18 10:20 +0200

#1622132 — Re: [PATCH] x86, kvm: Handle PFNs outside of kernel reach when touching GPTEs

Thanks for taking the time and sorry for the delay.

> On 6. Apr 2017, at 16:22, Radim Krčmář <rkrcmar@redhat.com> wrote:
> 
> 2017-04-05 15:07+0200, Filippo Sironi:
>> cmpxchg_gpte() calls get_user_pages_fast() to retrieve the number of
>> pages and the respective struct pages for mapping in the kernel virtual
>> address space.
>> This doesn't work if get_user_pages_fast() is invoked with a userspace
>> virtual address that's backed by PFNs outside of kernel reach (e.g.,
>> when limiting the kernel memory with mem= in the command line and using
>> /dev/mem to map memory).
>> 
>> If get_user_pages_fast() fails, look up the VMA that backs the userspace
>> virtual address, compute the PFN and the physical address, and map it in
>> the kernel virtual address space with memremap().
> 
> What is the reason for a configuration that voluntarily restricts access
> to memory that it needs?

By using /dev/mem to provide VM memory, one can avoid the overhead of allocating struct page(s) for the whole memory, which is wasteful when using a server entirely for hosting VMs.

>> Signed-off-by: Filippo Sironi <sironi@amazon.de>
>> Cc: Anthony Liguori <aliguori@amazon.com>
>> Cc: kvm@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> ---
>> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
>> @@ -147,15 +147,36 @@ static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>> 	struct page *page;
>> 
>> 	npages = get_user_pages_fast((unsigned long)ptep_user, 1, 1, &page);
>> -	/* Check if the user is doing something meaningless. */
>> -	if (unlikely(npages != 1))
>> -		return -EFAULT;
>> -
>> -	table = kmap_atomic(page);
>> -	ret = CMPXCHG(&table[index], orig_pte, new_pte);
>> -	kunmap_atomic(table);
>> -
>> -	kvm_release_page_dirty(page);
>> +	if (likely(npages == 1)) {
>> +		table = kmap_atomic(page);
>> +		ret = CMPXCHG(&table[index], orig_pte, new_pte);
>> +		kunmap_atomic(table);
>> +
>> +		kvm_release_page_dirty(page);
>> +	} else {
>> +		struct vm_area_struct *vma;
>> +		unsigned long vaddr = (unsigned long)ptep_user & PAGE_MASK;
>> +		unsigned long pfn;
>> +		unsigned long paddr;
>> +
>> +		down_read(&current->mm->mmap_sem);
>> +		vma = find_vma_intersection(current->mm, vaddr,
>> +					    vaddr + PAGE_SIZE);
> 
> Hm, with the argument order like this, we check that
> 
>  vaddr < vma->vm_end && vaddr + PAGE_SIZE > vma->vm_start
> 
> but shouldn't we actually check that the whole page is there, i.e.
> 
>  vaddr + PAGE_SIZE < vma->vm_end && vaddr > vma->vm_start
> 
> ?
> 
> Thanks.

Hm, right now we check for the following:

    vaddr >= vma->vm_start && vaddr < vma->vm_end && vaddr + PAGE_SIZE > vma->vm_start

given that vaddr is PAGE_SIZE aligned, we're guaranteed that vaddr + PAGE_SIZE <= vma->vm_end.
This seems more complex than necessary. I believe that:

    vma = find_vma(current->mm, vaddr);

should be enough.

>> +		if (!vma || !(vma->vm_flags & VM_PFNMAP)) {
>> +			up_read(&current->mm->mmap_sem);
>> +			return -EFAULT;
>> +		}
>> +		pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
>> +		paddr = pfn << PAGE_SHIFT;
>> +		table = memremap(paddr, PAGE_SIZE, MEMREMAP_WB);
> 
> (I don't undestand why there isn't a wrapper for this ...
> Looks like we're doing something unexpected.)

Do you mean a wrapper for getting the pfn/paddr?

>> +		if (!table) {
>> +			up_read(&current->mm->mmap_sem);
>> +			return -EFAULT;
>> +		}
>> +		ret = CMPXCHG(&table[index], orig_pte, new_pte);
>> +		memunmap(table);
>> +		up_read(&current->mm->mmap_sem);
>> +	}
>> 
>> 	return (ret != orig_pte);
>> }
>> -- 
>> 2.7.4

I'll submit a v2 version soon.

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#1624641

On 04/12/2017 09:16 PM, Sironi, Filippo wrote:
> Thanks for taking the time and sorry for the delay.
>
>> On 6. Apr 2017, at 16:22, Radim Krčmář <rkrcmar@redhat.com> wrote:
>>
>> 2017-04-05 15:07+0200, Filippo Sironi:
>>> cmpxchg_gpte() calls get_user_pages_fast() to retrieve the number of
>>> pages and the respective struct pages for mapping in the kernel virtual
>>> address space.
>>> This doesn't work if get_user_pages_fast() is invoked with a userspace
>>> virtual address that's backed by PFNs outside of kernel reach (e.g.,
>>> when limiting the kernel memory with mem= in the command line and using
>>> /dev/mem to map memory).
>>>
>>> If get_user_pages_fast() fails, look up the VMA that backs the userspace
>>> virtual address, compute the PFN and the physical address, and map it in
>>> the kernel virtual address space with memremap().
>>
>> What is the reason for a configuration that voluntarily restricts access
>> to memory that it needs?
>
> By using /dev/mem to provide VM memory, one can avoid the overhead of allocating struct page(s) for the whole memory, which is wasteful when using a server entirely for hosting VMs.
>

Sounds reasonable, however it is incomplete so far as there are some
code paths still do not support non-page backend memory, e.g,
emulator_cmpxchg_emulated().

I would suggest to unify the code introduced in this patch with existing
hva_to_pfn(), also we can introduce a common API, maybe named
kvm_map_hva(), to improve the caller sides.


BTW, i do not know why we used kmap_atomic() rather than kmap(), the
path of cmpxchg_gpte() is sleep-able anyway.

Thanks!

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#1625120

On 17/04/2017 13:26, Xiao Guangrong wrote:
>>
> 
> Sounds reasonable, however it is incomplete so far as there are some
> code paths still do not support non-page backend memory, e.g,
> emulator_cmpxchg_emulated().
> 
> I would suggest to unify the code introduced in this patch with existing
> hva_to_pfn(), also we can introduce a common API, maybe named
> kvm_map_hva(), to improve the caller sides.

I agree with Guangrong's suggestion.

Paolo

> BTW, i do not know why we used kmap_atomic() rather than kmap(), the
> path of cmpxchg_gpte() is sleep-able anyway.

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