Re: [Xen-devel] [PATCH v4] xen/arm: flush icache as well when XEN_DOMCTL_cacheflush is issued

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 27/01/17 23:53, Stefano Stabellini wrote:
> On Fri, 27 Jan 2017, Julien Grall wrote:
> > On 27/01/2017 20:41, Stefano Stabellini wrote:
> > > On Fri, 27 Jan 2017, Tamas K Lengyel wrote:
> > For the second instance, we have no other choice.
> Most alloc_heap_pages (alloc_xenheap_pages and alloc_domheap_pages) are
> done at domain initialization, so they would also be taken care by
> flushing the instruction cache before the domain is running. There are
> only very few exceptions to that, the main one being ballooning, and we
> need another icache flush in that case. But I think we should avoid
> doing global icache flushes every time alloc_heap_pages is called.
The invalidation of the full icache is unfortunate but necessary in 
non-PIPT cache. For PIPT cache you could do invalidation by VA.
Limiting the number of call is a nice optimization, but we need to be 
careful how to do it. Until then, a full icache invalidation (or by VA 
for PIPT) is the best solution.
> > > I am also wondering about all the libxc/libxl callers, many of whom
> > > don't need an icache flush. Ideally we would have an argument in
> > > XEN_DOMCTL_cacheflush to specify the type of cache flush we need,
> > > similar to GNTTABOP_cache_flush.
> > Unless the instruction cache will be flushed before the guest is booting, all
> > the callers of DOMCTL_cacheflush will require the invalidation.
> DOMCTL_cacheflush is called several time during the domain build, it is
> certainly better to do the icache flush once, rather than many times.
>
> If we decide to perform one icache flush at domain creation in Xen at
> the right time, we still need XEN_DOMCTL_cacheflush in its current form
> to flush the dcache.
>
> Also we still need a way to flush the icache to solve Tamas' problem
> with mem_access at run time.
>
> As a consequence, even after introducing one icache flush in Xen at
> domain creation, I think we still need a new "icache flush" flag in
> XEN_DOMCTL_cacheflush: all the current callers would not use it, but
> mem_access userspace components will be able to use it.
Why do we need a flag? No matter the way the flag is defined (set -> 
icache invalidation vs set -> no icache invalidation), a user will 
likely misuse it. The hypervisor is the best person to decide whether 
the icache flush is needed. Aside at domain building time, 99.9% (to not 
say 100%) of the call to cacheflush will require the invalidation of the 
icache.
Furthermore, if you implement the optimization for invalidating PIP 
icache (e.g by VA rather than full), a user will not have the 
possibility to invalidate the full icache.
So I would go the same way as it has been done for tlbflush (see bbb17f6 
"move TLB-flush filtering out into populate_physmap during vm 
creation"). Let Xen decides when to optimize the invalidation.
> > Looking at GNTTABOP_cache_flush, I think we will also need to invalidate the
> > instruction cache (or at least partially).
Cheers,

--
Julien Grall

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