Re: [PATCH V3 (resend) 06/19] x86: Add a boot option to enable and disable the direct map

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, May 15, 2024 at 03:54:51PM +0200, Jan Beulich wrote:
> On 14.05.2024 11:20, Roger Pau Monné wrote:
> > On Mon, May 13, 2024 at 01:40:33PM +0000, Elias El Yandouzi wrote:
> >> --- a/docs/misc/xen-command-line.pandoc
> >> +++ b/docs/misc/xen-command-line.pandoc
> >> @@ -799,6 +799,18 @@ that enabling this option cannot guarantee anything beyond what underlying
> >>  hardware guarantees (with, where available and known to Xen, respective
> >>  tweaks applied).
> >>  
> >> +### directmap (x86)
> >> +> `= <boolean>`
> >> +
> >> +> Default: `true`
> >> +
> >> +Enable or disable the directmap region in Xen.
> > 
> > Enable or disable fully populating the directmap region in Xen.
> 
> Elias, would you please take care to address earlier review comments
> before sending a new version? This and ...
> 
> >> +
> >> +By default, Xen creates the directmap region which maps physical memory
> >                                                           ^ all?
> >> +in that region. Setting this to no will sparsely populate the directmap,
> > 
> > "Setting this to no" => "Disabling this option will sparsely..."
> 
> ... this is what I had already asked for in reply to v2, of course with
> different wording.
> 
> >> --- a/xen/arch/x86/setup.c
> >> +++ b/xen/arch/x86/setup.c
> >> @@ -1517,6 +1517,8 @@ void asmlinkage __init noreturn __start_xen(unsigned long mbi_p)
> >>      if ( highmem_start )
> >>          xenheap_max_mfn(PFN_DOWN(highmem_start - 1));
> >>  
> >> +    printk("Booting with directmap %s\n", has_directmap() ? "on" : "off");
> > 
> > IMO this wants to be printed as part of the speculation mitigations, see
> > print_details() in spec_ctrl.c
> 
> And not "on" / "off", but "full" / "sparse" (and word order changed accordingly)
> perhaps.

I've been thinking about this, and I'm leaning towards calling this
new mode "ondemand" rather than "sparse".  The fact that the direct
map ends up sparely populated is a consequence of populating it on
demand, and hence the later would be more descriptive IMO.

(Same for the Kconfig option then ONDEMAND_DIRECTMAP, or some such)

Thanks, Roger.