Re: [Xen-devel] [PATCH] libxc: Add xc_domain_hvm_get_mtrr_type() call

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2012-12-19 at 10:42 +0000, Razvan Cojocaru wrote:
> > It would certainly be preferable to have libxc do it now rather than
> > every application it or having to introduce cleverer versions of the API
> > in the future.
> 
> I agree.
> 
> > I don't know enough about MTRRs or how Xen represents them either
> > internally or at the hypercall interface to have a sensible opinion
> > about the feasibility but my gut feeling is that there's no reason it
> > shouldn't be.
> 
> Unfortunately I'm far from being an expert on MTRRs too,

There is a section in the Intel SDM (chapter 10.4 in my copy) which
explains the meanings etc of the registers, which are what is exposed in
hvm_hw_mtrr I think.

>  so maybe 
> someone can tell us what the equivalent of this:
> 
> uint8_t get_mtrr_type(struct mtrr_state *m, paddr_t pa)
> {
>      [...]
>      if ( unlikely(!(m->enabled & 0x2)) )
>          return MTRR_TYPE_UNCACHABLE;
>      [...]
> }
> 
> whould be with libxc code using struct hvm_hw_mtrr? Or, more to the 
> point, what the equivalent of m->enabled is in this:

xen/arch/x86/hvm/mtrr.c:mtrr_def_type_msr_set() seems to be where it is
written, and it is called with hw_mtrr.msr_mtrr_def_type so it looks
like it can be derived from that value in hvm_hw_mtrr.

> 
> struct hvm_hw_mtrr {
> #define MTRR_VCNT 8
> #define NUM_FIXED_MSR 11
>      uint64_t msr_pat_cr;
>      /* mtrr physbase & physmask msr pair*/
>      uint64_t msr_mtrr_var[MTRR_VCNT*2];
>      uint64_t msr_mtrr_fixed[NUM_FIXED_MSR];
>      uint64_t msr_mtrr_cap;
>      uint64_t msr_mtrr_def_type;
> };
> 
> I'm also having a hard time figuring out how to map m->overlapped on the 
> hvm_hw_mtrr members.

    m->overlapped = is_var_mtrr_overlapped(m);

Looks like that function contains the necessary logic.

>  It's also quite possible (to me, at least, at this 
> stage) that they're not needed because at libxc level they're assumed to 
> be set to some value (i.e. 'overlapped' is assumed to be 0, 'enabled' to 
> be 1).
> 
> Any help is appreciated, if we clear this up I'll modify and re-submit 
> the patch.
> 
> Thanks,
> Razvan Cojocaru



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