Re: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

[Luck, Tony]

On Wed, Sep 29, 2021 at 09:58:22AM -0700, Andy Lutomirski wrote:
> On 9/28/21 16:10, Luck, Tony wrote:
> > Moving beyond pseudo-code and into compiles-but-probably-broken-code.
> > 
> > 
> > The intent of the functions below is that Fenghua should be able to
> > do:
> > 
> > void fpu__pasid_write(u32 pasid)
> > {
> > 	u64 msr_val = pasid | MSR_IA32_PASID_VALID;
> > 	struct ia32_pasid_state *addr;
> > 
> > 	addr = begin_update_one_xsave_feature(current, XFEATURE_PASID, true);
> > 	addr->pasid = msr_val;
> > 	finish_update_one_xsave_feature(current);
> > }
> > 
> 
> This gets gnarly because we would presumably like to optimize the case where
> we can do the update directly in registers.  I wonder if we can do it with a
> bit of macro magic in a somewhat generic way:

Can we defere the optimizations until there is a use case that
cares? The existing use case (fixing up the #GP fault by setting
the PASID MSR) isn't performance critical.

Let's just have something that is clear (or as clear as any xsave
code can be) and correct.

-Tony