Re: [PATCH kernel] KVM: PPC: Fix hardware and emulated TCE tables matching

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Jun 20, 2018 at 06:42:58PM +1000, Alexey Kardashevskiy wrote:
> When attaching a hardware table to LIOBN in KVM, we match table parameters
> such as page size, table offset and table size. However the tables are
> created via very different paths - VFIO and KVM - and the VFIO path goes
> through the platform code which has minimum TCE page size requirement
> (which is 4K but since we allocate memory by pages and cannot avoid
> alignment anyway, we align to 64k pages for powernv_defconfig).
> 
> So when we match the tables, one might be bigger that the other which
> means the hardware table cannot get attached to LIOBN and DMA mapping
> fails.
> 
> This removes the table size alignment from the guest visible table.
> This does not affect the memory allocation which is still aligned -
> kvmppc_tce_pages() takes care of this.
> 
> This relaxes the check we do when attaching tables to allow the hardware
> table be bigger than the guest visible table.
> 
> Ideally we want the KVM table to cover the same space as the hardware
> table does but since the hardware table may use multiple levels, and
> all levels must use the same table size (IODA2 design), the area it can
> actually cover might get very different from the window size which
> the guest requested, even though the guest won't map it all.
> 
> Fixes: ca1fc489cf "KVM: PPC: Book3S: Allow backing bigger guest IOMMU pages with smaller physical pages"
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  arch/powerpc/kvm/book3s_64_vio.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
> index 8c456fa..8167ce8 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -180,7 +180,7 @@ extern long kvm_spapr_tce_attach_iommu_group(struct kvm *kvm, int tablefd,
>  		if ((tbltmp->it_page_shift <= stt->page_shift) &&
>  				(tbltmp->it_offset << tbltmp->it_page_shift ==
>  				 stt->offset << stt->page_shift) &&
> -				(tbltmp->it_size << tbltmp->it_page_shift ==
> +				(tbltmp->it_size << tbltmp->it_page_shift >=
>  				 stt->size << stt->page_shift)) {
>  			/*
>  			 * Reference the table to avoid races with
> @@ -296,7 +296,7 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  {
>  	struct kvmppc_spapr_tce_table *stt = NULL;
>  	struct kvmppc_spapr_tce_table *siter;
> -	unsigned long npages, size;
> +	unsigned long npages, size = args->size;
>  	int ret = -ENOMEM;
>  	int i;
>  
> @@ -304,7 +304,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  		(args->offset + args->size > (ULLONG_MAX >> args->page_shift)))
>  		return -EINVAL;
>  
> -	size = _ALIGN_UP(args->size, PAGE_SIZE >> 3);
>  	npages = kvmppc_tce_pages(size);
>  	ret = kvmppc_account_memlimit(kvmppc_stt_pages(npages), true);
>  	if (ret)

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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