linux-kernel - Re: [PATCH 19/19] vfio/pci: Implement huge

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Message-ID: <20240814100849.601c14da.alex.williamson@redhat.com>
Date: Wed, 14 Aug 2024 10:08:49 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
 Oscar Salvador <osalvador@...e.de>, Axel Rasmussen
 <axelrasmussen@...gle.com>, linux-arm-kernel@...ts.infradead.org,
 x86@...nel.org, Will Deacon <will@...nel.org>, Gavin Shan
 <gshan@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>, Zi Yan
 <ziy@...dia.com>, Andrew Morton <akpm@...ux-foundation.org>, Catalin
 Marinas <catalin.marinas@....com>, Ingo Molnar <mingo@...hat.com>, Alistair
 Popple <apopple@...dia.com>, Borislav Petkov <bp@...en8.de>, David
 Hildenbrand <david@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
 kvm@...r.kernel.org, Dave Hansen <dave.hansen@...ux.intel.com>, Yan Zhao
 <yan.y.zhao@...el.com>
Subject: Re: [PATCH 19/19] vfio/pci: Implement huge_fault support

On Wed, 14 Aug 2024 10:25:08 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Fri, Aug 09, 2024 at 12:09:09PM -0400, Peter Xu wrote:
> > @@ -1672,30 +1679,49 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
> >  	if (vdev->pm_runtime_engaged || !__vfio_pci_memory_enabled(vdev))
> >  		goto out_unlock;
> >  
> > -	ret = vmf_insert_pfn(vma, vmf->address, pfn + pgoff);
> > -	if (ret & VM_FAULT_ERROR)
> > -		goto out_unlock;
> > -
> > -	/*
> > -	 * Pre-fault the remainder of the vma, abort further insertions and
> > -	 * supress error if fault is encountered during pre-fault.
> > -	 */
> > -	for (; addr < vma->vm_end; addr += PAGE_SIZE, pfn++) {
> > -		if (addr == vmf->address)
> > -			continue;
> > -
> > -		if (vmf_insert_pfn(vma, addr, pfn) & VM_FAULT_ERROR)
> > -			break;
> > +	switch (order) {
> > +	case 0:
> > +		ret = vmf_insert_pfn(vma, vmf->address, pfn + pgoff);
> > +		break;
> > +#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
> > +	case PMD_ORDER:
> > +		ret = vmf_insert_pfn_pmd(vmf, __pfn_to_pfn_t(pfn + pgoff,
> > +							     PFN_DEV), false);
> > +		break;
> > +#endif
> > +#ifdef CONFIG_ARCH_SUPPORTS_PUD_PFNMAP
> > +	case PUD_ORDER:
> > +		ret = vmf_insert_pfn_pud(vmf, __pfn_to_pfn_t(pfn + pgoff,
> > +							     PFN_DEV), false);
> > +		break;
> > +#endif  
> 
> I feel like this switch should be in some general function? 
> 
> vmf_insert_pfn_order(vmf, order, __pfn_to_pfn_t(pfn + pgoff, PFN_DEV), false);
> 
> No reason to expose every driver to this when you've already got a
> nice contract to have the driver work on the passed in order.
> 
> What happens if the driver can't get a PFN that matches the requested
> order?

There was some alignment and size checking chopped from the previous
reply that triggered a fallback, but in general PCI BARs are a power of
two and naturally aligned, so there should always be an order aligned
pfn.  Thanks,

Alex