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Re: [PATCH 3/5] x86/PV: _PAGE_RW changes may take fast path of mod_l[234]_entry()
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 11 Jan 2021 15:26:03 +0100
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On Mon, Jan 11, 2021 at 02:31:10PM +0100, Jan Beulich wrote:
> On 11.01.2021 12:08, Roger Pau Monné wrote:
> > On Tue, Nov 03, 2020 at 11:57:10AM +0100, Jan Beulich wrote:
> >> The only time _PAGE_RW matters when validating an L2 or higher entry is
> >> when a linear page table is tried to be installed. Therefore when we
> >> disallow such at build time, we can allow _PAGE_RW changes to take the
> >> fast paths there.
> >
> > I think it would be helpful to note why PDEs with linear page tables
> > aren't allowed to have the RW bit set, likely here and in the comment
> > below.
>
> I've changed the description to
>
> "The only time _PAGE_RW matters when validating an L2 or higher entry is
> when a linear page table is tried to be installed (see the comment ahead
> of define_get_linear_pagetable()). Therefore when we disallow such at
> build time, we can allow _PAGE_RW changes to take the fast paths there."
>
> Considering there already is a code comment explaining this, I'm
> less convinced of also editing the comment. An option might be to
> move up the #define-s next to define_get_linear_pagetable(), but
> imo this should then involve moving FASTPATH_FLAG_WHITELIST as
> well.
Referencing the comment in define_get_linear_pagetable in the commit
message seems fine to me:
Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Thanks, Roger.
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