Re: [PATCH v3 2/2] x86/ept: limit calls to memory_type_changed()

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Han,

> On 29 Sep 2022, at 12:13, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
> On 28.09.2022 16:11, 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.
>> 
>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> with one minor remark at the end, which can be taken care of while committing.
> 
>> ---
>> Changes since v2:
>> - Split the Arm side changes into a pre-patch.
> 
> Despite this I'd prefer to have an Arm maintainer view on this as well. As
> previously pointed out the resulting code is going to be sub-optimal there.

On arm none of those will be called at runtime, it happens only during guest 
creation
so the potential performance impact is very reduce.

Cheers
Bertrand

> 
>> --- 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>
>> +
>> +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,
> 
> A blank line would be nice between these two (and similarly for the
> x86-only pair). Omitting such blank lines is imo advisable only for
> trivial inline functions.
> 
> Jan