Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set

[Paolo Bonzini]

On 18/07/2016 19:07, Sergey Fedorov wrote:
> On 18/07/16 20:00, Paolo Bonzini wrote:
>>
>> On 18/07/2016 18:57, Sergey Fedorov wrote:
>>> On 18/07/16 19:53, Paolo Bonzini wrote:
>>>> On 18/07/2016 18:52, Sergey Fedorov wrote:
>>>>> So how are we going to use them?
>>>> Instead of atomic_read/atomic_set when marking invalid TBs.
>>> But shouldn't they be atomic to avoid reading torn writes?
>> A torn write would probably fail to match anyway, but even if it doesn't
>> it is indistinguishable from a race, isn't it?
> 
> I'm afraid, torn write can happen to be a false match against a wrong
> TB. In case of a race with atomic access we either get the right TB or
> an invalid one which couldn't match any valid CPU state. Probably, we
> have to make sure (and document this) that TB invalidation process
> cannot make a partially invalidated TB which can match any meaningful
> CPU state.

x86 is atomic (because flags are 32-bit); those that have cs_base==0 are
safe against torn writes too.  Only SPARC perhaps could use
"tb->cs_base|=1" instead in case 0xffffffff........ matches another TB.

Paolo

>>
>> By the way, tb_cmp should also use volatile_read.
> 
> You are right, we must user the save type of access in tb_cmp().
> 
> Thanks,
> Sergey
>