[PATCH] arm/arm64: KVM: introduce new mapping API for percpu mappings

[christoffer.dall@linaro.org (Christoffer Dall)]

On Fri, Nov 15, 2013 at 04:59:53PM +0000, Marc Zyngier wrote:
> On 15/11/13 16:43, Christoffer Dall wrote:
> > On Fri, Nov 15, 2013 at 04:33:07PM +0000, Marc Zyngier wrote:
> >> On 15/11/13 16:10, Christoffer Dall wrote:
> >>> On Fri, Nov 15, 2013 at 03:40:08PM +0000, Marc Zyngier wrote:
> >>>> Using virt_to_phys on percpu mappings is horribly wrong (my own bad).
> >>>> Thankfully, the kernel offers a way to obtain the physical address
> >>>> of such a mapping.
> >>>>
> >>>> Add a new create_hyp_percpu_mappings function to deal with those.
> >>>>
> >>>> Reported-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
> >>>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
> >>>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >>>> ---
> >>>
> >>>
> >>> So, I find this nicer, somehow, what do you think:
> >>>
> >>>
> >>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> >>> index 3719583..dd531ba 100644
> >>> --- a/arch/arm/kvm/mmu.c
> >>> +++ b/arch/arm/kvm/mmu.c
> >>> @@ -334,6 +334,15 @@ out:
> >>>  	return err;
> >>>  }
> >>>  
> >>> +static phys_addr_t kvm_kaddr_to_phys(void *kaddr)
> >>> +{
> >>> +	if (!is_vmalloc_addr(kaddr))
> >>> +		return __pa(kaddr);
> >>> +	else
> >>> +		return page_to_phys(vmalloc_to_page(kaddr)) +
> >>> +		       offset_in_page(kaddr);
> >>> +}
> >>> +
> >>>  /**
> >>>   * create_hyp_mappings - duplicate a kernel virtual address range in Hyp mode
> >>>   * @from:	The virtual kernel start address of the range
> >>> @@ -345,16 +354,24 @@ out:
> >>>   */
> >>>  int create_hyp_mappings(void *from, void *to)
> >>>  {
> >>> -	unsigned long phys_addr = virt_to_phys(from);
> >>> +	phys_addr_t phys_addr;
> >>> +	unsigned long virt_addr;
> >>>  	unsigned long start = KERN_TO_HYP((unsigned long)from);
> >>>  	unsigned long end = KERN_TO_HYP((unsigned long)to);
> >>>  
> >>> -	/* Check for a valid kernel memory mapping */
> >>> -	if (!virt_addr_valid(from) || !virt_addr_valid(to - 1))
> >>> -		return -EINVAL;
> >>> +	for (virt_addr = start; virt_addr < end; virt_addr += PAGE_SIZE) {
> >>> +		int err;
> >>>  
> >>> -	return __create_hyp_mappings(hyp_pgd, start, end,
> >>> -				     __phys_to_pfn(phys_addr), PAGE_HYP);
> >>> +		phys_addr = kvm_kaddr_to_phys(from + virt_addr - start);
> >>> +		err = __create_hyp_mappings(hyp_pgd, virt_addr,
> >>> +					    virt_addr + PAGE_SIZE,
> >>
> >> I think I've introduced a bug here. It probably should read:
> >>
> >> 	err = __create_hyp_mappings(hyp_pgd, virt_addr & PAGE_MASK,
> >> 				    (virt_addr + PAGE_SIZE) & PAGE_MASK,
> >> 				    [...]
> >>
> >>> +					    __phys_to_pfn(phys_addr),
> >>> +					    PAGE_HYP);
> >>> +		if (err)
> >>> +			return err;
> >>> +	}
> >>> +
> >>> +	return 0;
> >>>  }
> >>>  
> >>>  /**
> >>>
> >>
> >> So that would work, but I'm slightly uncomfortable with what is
> >> basically an open-coded version of per_cpu_ptr_to_phys, and I think
> >> there is some value in having an explicit function for dealing with
> >> percpu mappings, at least for educational purpose.
> > 
> > I'm not concerned about the open-coded; this is a pretty fundamental
> > functionality to distinguish between a vmalloc and a kmalloc and do the
> > virt_to_phys translation accordingly - that's not specific to percpu
> > allocated memory - they only add more complexity as an optimization.
> > After all, this is a single if-statement that I'm sure we can master.
> 
> Hey, I've been seen messing up that kind of code very easily! ;-)
> 
> > I think it's slightly more strange to have a call to map memory that
> > depends on how you allocated it, and would prefer having something that
> > just works, regardless.  But maybe that's utopian.  However, would we
> > end up potentially having a create_vmalloc_hyp_mappings as well then?
> > 
> > Another concern with your proposal is that it duplicates more code and
> > makes it a bit harder to track what's going on (who calls what when to
> > allocate something, etc.).
> > 
> >>
> >> Also, we loose the virt_addr_valid() check, which has been a valuable
> >> debugging tool for me in the past.
> >>
> >> But maybe that's just me being a chicken... ;-)
> >>
> > Of course it is.  No, but really, virt_addr_valid just checks that it's
> > linearly mapped mapped memory.  So we're checking that it's not
> > highmeme, and assuming it's linearly mapped now.  We can add a
> > BUG_ON(!virt_addr_valid(x)) in the (!is_vmalloc_addr(kaddr)) case to
> > make sure nobody is passing module addresses or DMA memory or something
> > else crazy here, but I doubt that's a bug we need to concerne ourselves
> > about.
> 
> Fair enough. Do you write the patch, or do I update mine? Don't mind
> either way.
> 
I'll write it and send it out.

Thanks,
-Christoffer