Re: [Xen-devel] [PATCH v8 9/9] xen/x86: use PCID feature

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.04.18 at 10:30, <jgross@xxxxxxxx> wrote:
> Avoid flushing the complete TLB when switching %cr3 for mitigation of
> Meltdown by using the PCID feature if available.
> 
> We are using 4 PCID values for a 64 bit pv domain subject to XPTI and
> 2 values for the non-XPTI case:
> 
> - guest active and in kernel mode
> - guest active and in user mode
> - hypervisor active and guest in user mode (XPTI only)
> - hypervisor active and guest in kernel mode (XPTI only)
> 
> We use PCID only if PCID _and_ INVPCID are supported. With PCID in use
> we disable global pages in cr4. A command line parameter controls in
> which cases PCID is being used.
> 
> As the non-XPTI case has shown not to perform better with PCID at least
> on some machines the default is to use PCID only for domains subject to
> XPTI.
> 
> With PCID enabled we always disable global pages. This avoids having to
> either flush the complete TLB or do a cycle through all PCID values
> when invalidating a single global page.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> V6.1:
> - address some minor comments (Jan Beulich)

No v7 changes? Afaict ...

> @@ -717,7 +718,7 @@ int __init dom0_construct_pv(struct domain *d,
>          update_cr3(v);
>  
>      /* We run on dom0's page tables for the final part of the build process. 
> */
> -    switch_cr3_cr4(v->arch.cr3, read_cr4());
> +    switch_cr3_cr4(cr3_pa(v->arch.cr3), read_cr4());

... at least this was added. And to be honest I'm not convinced cr3_pa() is
the right construct to use here: It's neither clear why the other bits don't
matter at this point, nor why any of the bits that you mask out this way
need masking out in the first place (e.g. why, in the crash case that Andrew
had observed, the noflush bit was wrongly set).

I hope there aren't any other such hidden changes in this version of the
series.

Jan



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