Re: [PATCH 3/5] x86/PV: _PAGE_RW changes may take fast path of mod_l[234]_entry()

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan