Re: [Xen-devel] [PATCH v10 4/6] VT-d: introduce update_irte to update irte safely

[Chao Gao <chao.gao@xxxxxxxxx>]

On Wed, Mar 15, 2017 at 10:48:25AM -0600, Jan Beulich wrote:
>>>> On 15.03.17 at 06:11, <chao.gao@xxxxxxxxx> wrote:
>> +        /*
>> +         * The following method to update IRTE is safe on condition that
>> +         * only the high qword or the low qword is to be updated.
>> +         * If entire IRTE is to be updated, callers should make sure the
>> +         * IRTE is not in use.
>> +         */
>> +        entry->lo = new_ire->lo;
>> +        entry->hi = new_ire->hi;
>
>How is this any better than structure assignment? Furthermore

Indeed, not better. when using structure assignment, the assembly code is
48 8b 06                mov    (%rsi),%rax                    
48 8b 56 08             mov    0x8(%rsi),%rdx                 
48 89 07                mov    %rax,(%rdi)                    
48 89 57 08             mov    %rdx,0x8(%rdi)
Using the code above, the assembly code is
48 8b 06                mov    (%rsi),%rax                  
48 89 07                mov    %rax,(%rdi)                    
48 8b 46 08             mov    0x8(%rsi),%rax                 
48 89 47 08             mov    %rax,0x8(%rdi)

I thought structure assignment maybe ultilize memcpy considering structure
of a big size, so I made this change. I will change this back. Although
that, this patch is trying to make the change safer when cmpxchg16() is
supported. 

>the comment here partially contradicts the commit message. I

Yes.

>guess callers need to be given a way (another function parameter?)
>to signal the function whether the unsafe variant is okay to use.

This means we need to add the new parameter to iommu ops for only
IOAPIC/MSI know the entry they want to change is masked. Is there
any another reasonable and correct solution? How about...

>You should then add a suitable BUG_ON() in the else path here.

just add a BUG_ON() like this
BUG_ON( (entry->hi != new_ire->hi) && (entry->lo != new_ire->lo) );
Adding this BUG_ON() means update_irte() can't be used for initializing
or clearing IRTE which are not bugs.

Thanks
Chao
>
>Jan
>

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