Re: [PATCH v2] x86/ept: limit calls to memory_type_changed()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Sep 28, 2022 at 10:01:26AM +0200, Jan Beulich wrote:
> On 27.09.2022 17:39, Roger Pau Monne wrote:
> > memory_type_changed() is currently only implemented for Intel EPT, and
> > results in the invalidation of EMT attributes on all the entries in
> > the EPT page tables.  Such invalidation causes EPT_MISCONFIG vmexits
> > when the guest tries to access any gfns for the first time, which
> > results in the recalculation of the EMT for the accessed page.  The
> > vmexit and the recalculations are expensive, and as such should be
> > avoided when possible.
> > 
> > Remove the call to memory_type_changed() from
> > XEN_DOMCTL_memory_mapping: there are no modifications of the
> > iomem_caps ranges anymore that could alter the return of
> > cache_flush_permitted() from that domctl.
> > 
> > Encapsulate calls to memory_type_changed() resulting from changes to
> > the domain iomem_caps or ioport_caps ranges in the helpers themselves
> > (io{ports,mem}_{permit,deny}_access()), and add a note in
> > epte_get_entry_emt() to remind that changes to the logic there likely
> > need to be propagaed to the IO capabilities helpers.
> > 
> > Note changes to the IO ports or memory ranges are not very common
> > during guest runtime, but Citrix Hypervisor has an use case for them
> > related to device passthrough.
> > 
> > Some Arm callers (implementations of the iomem_deny_access function
> > pointer field in gic_hw_operations struct) pass a const domain pointer
> > to iomem_deny_access(), which is questionable.  It works because
> > the rangeset is allocated separately from the domain struct, but
> > conceptually seems wrong to me, as passing a const pointer would imply
> > no changes to the domain data, and denying iomem accesses does change
> > the domain data.  Fix this by removing the const attribute from the
> > affected functions and call chain.
> 
> Personally I think this adjustment would better be a separate, prereq
> change.

Right - I was about to split it but didn't want to go through the
hassle if the approach didn't end up being well received.

Do you think placing the calls to memory_type_changed() inside the
{permit,deny}_,access is acceptable?

> > --- a/xen/include/xen/iocap.h
> > +++ b/xen/include/xen/iocap.h
> > @@ -7,13 +7,43 @@
> >  #ifndef __XEN_IOCAP_H__
> >  #define __XEN_IOCAP_H__
> >  
> > +#include <xen/sched.h>
> >  #include <xen/rangeset.h>
> >  #include <asm/iocap.h>
> > +#include <asm/p2m.h>
> 
> That's heavy dependencies you're adding. I wonder if the functions
> wouldn't better become out-of-line ones (but see also below).
> 
> > +static inline int iomem_permit_access(struct domain *d, unsigned long s,
> > +                                      unsigned long e)
> > +{
> > +    bool flush = cache_flush_permitted(d);
> > +    int ret = rangeset_add_range(d->iomem_caps, s, e);
> > +
> > +    if ( !ret && !is_iommu_enabled(d) && !flush )
> > +        /*
> > +         * Only flush if the range(s) are empty before this addition and
> > +         * IOMMU is not enabled for the domain, otherwise it makes no
> > +         * difference for effective cache attribute calculation purposes.
> > +         */
> > +        memory_type_changed(d);
> > +
> > +    return ret;
> > +}
> > +static inline int iomem_deny_access(struct domain *d, unsigned long s,
> > +                                    unsigned long e)
> > +{
> > +    int ret = rangeset_remove_range(d->iomem_caps, s, e);
> > +
> > +    if ( !ret && !is_iommu_enabled(d) && !cache_flush_permitted(d) )
> > +        /*
> > +         * Only flush if the range(s) are empty after this removal and
> > +         * IOMMU is not enabled for the domain, otherwise it makes no
> > +         * difference for effective cache attribute calculation purposes.
> > +         */
> > +        memory_type_changed(d);
> > +
> > +    return ret;
> > +}
> 
> I'm surprised Arm's memory_type_changed() is an empty out-of-line function.
> This means the compiler can't eliminate this code (except when using LTO).
> But then cache_flush_permitted() (resolving to rangeset_is_empty()) can't
> be eliminated either, even if memory_type_changed() was. While gcc doc
> doesn't explicitly say that it may help (the talk about repeated invocations
> only), I wonder whether we shouldn't mark rangeset_is_empty() pure. In a
> reduced example that does help (once memory_type_changed() is also an
> inline function) with gcc12 - no call to rangeset_is_empty() remains.

Can look into it, do you want it to be a prereq of this patch?

Thanks, Roger.