linux-kernel - Re: [PATCH] powerpc/32s: Don't flush all TLBs when flushing one page

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Message-ID: <20200131193833.GF22482@gate.crashing.org>
Date:   Fri, 31 Jan 2020 13:38:33 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/32s: Don't flush all TLBs when flushing one page

On Fri, Jan 31, 2020 at 05:15:20PM +0100, Christophe Leroy wrote:
> Le 31/01/2020 à 16:51, Segher Boessenkool a écrit :
> >On Fri, Jan 31, 2020 at 03:37:34PM +0000, Christophe Leroy wrote:
> >>When the range is a single page, do a page flush instead.
> >
> >>+	start &= PAGE_MASK;
> >>+	end = (end - 1) | ~PAGE_MASK;
> >>  	if (!Hash) {
> >>-		_tlbia();
> >>+		if (end - start == PAGE_SIZE)
> >>+			_tlbie(start);
> >>+		else
> >>+			_tlbia();
> >>  		return;
> >>  	}
> >
> >For just one page, you get  end - start == 0  actually?
> 
> Oops, good catch.
> 
> Indeed you don't get PAGE_SIZE but (PAGE_SIZE - 1) for just one page.

You have all low bits masked off in both start and end, so you get zero.
You could make the condion read "if (start == end)?

Maybe a nicer way to describe what you do is "if start and end are on the
same memory page, flush that page."


Segher