Re: [Xen-devel] [PATCH v4 4/7] xen/x86: use invpcid for flushing the TLB

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.18 at 11:07, <jgross@xxxxxxxx> wrote:
> --- a/xen/arch/x86/setup.c
> +++ b/xen/arch/x86/setup.c
> @@ -63,6 +63,10 @@ boolean_param("nosmp", opt_nosmp);
>  static unsigned int __initdata max_cpus;
>  integer_param("maxcpus", max_cpus);
>  
> +/* opt_invpcid: If false, don't use INVPCID instruction even if available. */
> +static bool __initdata opt_invpcid = true;
> +boolean_param("invpcid", opt_invpcid);

Hmm, I'm sorry for noticing only now (while seeing the questionable
uses of cpu_has_invpcid in patch 7), but this being an init-only
variable and having ...

> @@ -1549,6 +1553,9 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>      if ( cpu_has_fsgsbase )
>          set_in_cr4(X86_CR4_FSGSBASE);
>  
> +    if ( !opt_invpcid )
> +        setup_clear_cpu_cap(X86_FEATURE_INVPCID);

... this effect has two issues: For one, in such a case this should
be a sub-option to "cpuid=". And then afaict it also disables use of
INVPCID in HVM guests. IOW I think you want to retain the option
but make the variable non-init and non-static. Obviously for early
boot use it may then no longer be possible to set it to true at build
time (you may end up needing two variables).

Jan


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