From mboxrd@z Thu Jan  1 00:00:00 1970
From: Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v8 11/27] x86/mm: Introduce _PAGE_DIRTY_SW
Date: Wed, 28 Aug 2019 07:57:41 -0700
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To: Peter Zijlstra <peterz@infradead.org>
Cc: x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>, Andy Lutomirski <luto@amacapital.net>, Balbir Singh <bsingharora@gmail.com>, Borislav Petkov <bp@alien8.de>, Cyrill Gorcunov <gorcunov@gmail.com>, Dave Hansen <dave.hansen@linux.intel.com>, Eugene Syromiatnikov <esyr@redhat.com>, Florian Weimer <fweimer@redhat.com>, "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>, Kees Cook <keescook@chromium.org>, Mike Kravetz <mike.kravetz@oracle.com>, Nadav Amit <nadav.amit@gmail.com>
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On Wed, 2019-08-28 at 09:03 +0200, Peter Zijlstra wrote:
> On Tue, Aug 27, 2019 at 03:37:12PM -0700, Yu-cheng Yu wrote:
> > On Fri, 2019-08-23 at 16:02 +0200, Peter Zijlstra wrote:
> > > On Tue, Aug 13, 2019 at 01:52:09PM -0700, Yu-cheng Yu wrote:
> > > 
> > > > +static inline pte_t pte_move_flags(pte_t pte, pteval_t from, pteval_t
> > > > to)
> > > > +{
> > > > +	if (pte_flags(pte) & from)
> > > > +		pte = pte_set_flags(pte_clear_flags(pte, from), to);
> > > > +	return pte;
> > > > +}
> > > 
> > > Aside of the whole conditional thing (I agree it would be better to have
> > > this unconditionally); the function doesn't really do as advertised.
> > > 
> > > That is, if @from is clear, it doesn't endeavour to make sure @to is
> > > also clear.
> > > 
> > > Now it might be sufficient, but in that case it really needs a comment
> > > and or different name.
> > > 
> > > An implementation that actually moves the bit is something like:
> > > 
> > > 	pteval_t a,b;
> > > 
> > > 	a = native_pte_value(pte);
> > > 	b = (a >> from_bit) & 1;
> > > 	a &= ~((1ULL << from_bit) | (1ULL << to_bit));
> > > 	a |= b << to_bit;
> > > 	return make_native_pte(a);
> > 
> > There can be places calling pte_wrprotect() on a PTE that is already RO +
> > DIRTY_SW.  Then in pte_move_flags(pte, _PAGE_DIRTY_HW, _PAGE_DIRTY_SW) we do
> > not
> >  want to clear _PAGE_DIRTY_SW.  But, I will look into this and make it more
> > obvious.
> 
> Well, then the name 'move' is just wrong, because that is not the
> semantics you're looking for.
> 
> So the thing is; if you provide a generic function that 'munges' two
> bits, then it's name had better be accurate. But AFAICT you only ever
> used this for the DIRTY bits, so it might be better to have a function
> specifically for that and with a comment that spells out the exact
> semantics and reasons for them.

Yes, I will work on that.

Yu-cheng