Re: [RFC PATCH v2 11/12] x86/mm/tlb: Use async and inline messages for flushing

[Andy Lutomirski]

On May 31, 2019, at 2:33 PM, Nadav Amit <namit@xxxxxxxxxx> wrote:

>> On May 31, 2019, at 2:14 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> 
>>> On Thu, May 30, 2019 at 11:37 PM Nadav Amit <namit@xxxxxxxxxx> wrote:
>>> When we flush userspace mappings, we can defer the TLB flushes, as long
>>> the following conditions are met:
>>> 
>>> 1. No tables are freed, since otherwise speculative page walks might
>>>  cause machine-checks.
>>> 
>>> 2. No one would access userspace before flush takes place. Specifically,
>>>  NMI handlers and kprobes would avoid accessing userspace.
>> 
>> I think I need to ask the big picture question.  When someone calls
>> flush_tlb_mm_range() (or the other entry points), if no page tables
>> were freed, they want the guarantee that future accesses (initiated
>> observably after the flush returns) will not use paging entries that
>> were replaced by stores ordered before flush_tlb_mm_range().  We also
>> need the guarantee that any effects from any memory access using the
>> old paging entries will become globally visible before
>> flush_tlb_mm_range().
>> 
>> I'm wondering if receipt of an IPI is enough to guarantee any of this.
>> If CPU 1 sets a dirty bit and CPU 2 writes to the APIC to send an IPI
>> to CPU 1, at what point is CPU 2 guaranteed to be able to observe the
>> dirty bit?  An interrupt entry today is fully serializing by the time
>> it finishes, but interrupt entries are epicly slow, and I don't know
>> if the APIC waits long enough.  Heck, what if IRQs are off on the
>> remote CPU?  There are a handful of places where we touch user memory
>> with IRQs off, and it's (sadly) possible for user code to turn off
>> IRQs with iopl().
>> 
>> I *think* that Intel has stated recently that SMT siblings are
>> guaranteed to stop speculating when you write to the APIC ICR to poke
>> them, but SMT is very special.
>> 
>> My general conclusion is that I think the code needs to document what
>> is guaranteed and why.
> 
> I think I might have managed to confuse you with a bug I made (last minute
> bug when I was doing some cleanup). This bug does not affect the performance
> much, but it might led you to think that I use the APIC sending as
> synchronization.
> 
> The idea is not for us to rely on write to ICR as something serializing. The
> flow should be as follows:
> 
> 
>    CPU0                    CPU1
> 
> flush_tlb_mm_range()
> __smp_call_function_many()
>  [ prepare call_single_data (csd) ]
>  [ lock csd ] 
>  [ send IPI ]
>    (*)
>  [ wait for csd to be unlocked ]
>                    [ interrupt ]
>                    [ copy csd info to stack ]
>                    [ csd unlock ]
>  [ find csd is unlocked ]
>  [ continue (**) ]
>                    [ flush TLB ]
> 
> 
> At (**) the pages might be recycled, written-back to disk, etc. Note that
> during (*), CPU0 might do some local TLB flushes, making it very likely that
> CSD will be unlocked by the time it gets there.
> 
> As you can see, I donât rely on any special micro-architectural behavior.
> The synchronization is done purely in software.
> 
> Does it make more sense now?
> 

Yes.  Have you benchmarked this?