Re: [PATCH v5 11/15] x86/mtrr: construct a memory map with cache modes

[Juergen Gross <jgross@suse.com>]

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On 20.04.23 16:54, Borislav Petkov wrote:
> On Thu, Apr 20, 2023 at 03:57:43PM +0200, Juergen Gross wrote:
>> So you are suggesting that prefetching can happen across one wrong speculated
>> branch, but not across two of them? And you are not worrying about prefetches
>> past the end of a copy with size > 0?
> 
> Maybe it will, maybe it won't.
> 
> I am worried about calling a function unnecessarily. I'm worried about
> calling the asm version __memmove() with zero length unnecessarily. I'm
> worried about executing the return thunk unnecessarily:
> 
> ffffffff8104c749:       48 83 c4 28             add    $0x28,%rsp
> ffffffff8104c74d:       e9 72 2a 9b 00          jmp    ffffffff819ff1c4 <__x86_return_thunk>
> ffffffff8104c752:       31 c0                   xor    %eax,%eax
> ffffffff8104c754:       e9 6b 2a 9b 00          jmp    ffffffff819ff1c4 <__x86_return_thunk>
> ffffffff8104c759:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
> 
> Just say that you don't want to do this simple check and I will do it
> myself because I'm tired of debating.

I just want to make sure to understand your concerns and that the reasoning
is sane.

You seem to feel rather strong here, so I'll add the test.

> 
>> "If two or more variable memory ranges match and one of the memory types is UC,
>> the UC memory type used."
>>
>> So technically no problem, apart from lower performance.
> 
> How do you come from "Write-combining to UC memory is not allowed" to
> "lower performance"?
> 
> Not allowed is not allowed. Geez.

Yes. And using UC instead of WC usually means lower performance of writes.

>> Would you be fine with adding that as an additional patch?
>>
>> I believe if we really want that, then we should be able to disable such a
>> cleanup. So it should be an add-on anyway.
> 
> Sure, whatever.

Okay, thanks.

I think this will need another final loop over the MTRRs to check against the
constructed map if a MTRR is completely useless.

Another question: in case we detect such a hidden MTRR, should it be disabled
in order to have more MTRRs available for run-time adding?

> 
>> I'm not against adding such additional checks. I wouldn't like to force them
>> into this series right now, because we need this series for Hyper-V isolated
>> guest support.
> 
> We will add this series when they're ready. If Hyper-V needs them
> immediately they can take whatever they want and do whatever they want.
> 
> Or you can do a simpler fix for Hyper-V that goes before this, if you
> want to address Hyper-V.
> 
>> Just to say it explicitly: you are concerned for the case that a complete
>> MTRR is hidden beneath another one (e.g. a large UC MTRR hiding a smaller
>> MTRR with another type, or a variable MTRR being hidden by fixed MTRRs)?
> 
> I am concerned about catching any and all inconsistencies with the MTRRs
> and catching them right. If we're going to spend all this time on this,
> then let's do it right, once and for all and do it in a manner that can
> be improved in the future.

Okay.


Juergen


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