Subject: Re: [PATCH kernel 3/9] KVM: PPC: Rework H_PUT_TCE/H_GET_TCE handlers

From: David Gibson <[email-masked]>
Date: Tue, 8 Dec 2015 13:18:04 +1100

On Tue, Sep 15, 2015 at 08:49:33PM +1000, Alexey Kardashevskiy wrote:
> This reworks the existing H_PUT_TCE/H_GET_TCE handlers to have one
> exit path. This allows next patch to add locks nicely.

I don't see a problem with the actual code, but it doesn't seem to
match this description: I still see multiple return statements for
h_put_tce at least.

> This moves the ioba boundaries check to a helper and adds a check for
> least bits which have to be zeros.
> 
> The patch is pretty mechanical (only check for least ioba bits is added)
> so no change in behaviour is expected.
> 
> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_64_vio_hv.c | 102 +++++++++++++++++++++++-------------
>  1 file changed, 66 insertions(+), 36 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_vio_hv.c b/arch/powerpc/kvm/book3s_64_vio_hv.c
> index 89e96b3..8ae12ac 100644
> --- a/arch/powerpc/kvm/book3s_64_vio_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_vio_hv.c
> @@ -35,71 +35,101 @@
>  #include <asm/ppc-opcode.h>
>  #include <asm/kvm_host.h>
>  #include <asm/udbg.h>
> +#include <asm/iommu.h>
>  
>  #define TCES_PER_PAGE	(PAGE_SIZE / sizeof(u64))
>  
> +/*
> + * Finds a TCE table descriptor by LIOBN.
> + *
> + * WARNING: This will be called in real or virtual mode on HV KVM and virtual
> + *          mode on PR KVM
> + */
> +static struct kvmppc_spapr_tce_table *kvmppc_find_table(struct kvm_vcpu *vcpu,
> +		unsigned long liobn)
> +{
> +	struct kvm *kvm = vcpu->kvm;
> +	struct kvmppc_spapr_tce_table *stt;
> +
> +	list_for_each_entry_rcu_notrace(stt, &kvm->arch.spapr_tce_tables, list)
> +		if (stt->liobn == liobn)
> +			return stt;
> +
> +	return NULL;
> +}
> +
> +/*
> + * Validates IO address.
> + *
> + * WARNING: This will be called in real-mode on HV KVM and virtual
> + *          mode on PR KVM
> + */
> +static long kvmppc_ioba_validate(struct kvmppc_spapr_tce_table *stt,
> +		unsigned long ioba, unsigned long npages)
> +{
> +	unsigned long mask = (1ULL << IOMMU_PAGE_SHIFT_4K) - 1;
> +	unsigned long idx = ioba >> IOMMU_PAGE_SHIFT_4K;
> +	unsigned long size = stt->window_size >> IOMMU_PAGE_SHIFT_4K;
> +
> +	if ((ioba & mask) || (size + npages <= idx))
> +		return H_PARAMETER;

Not sure if it's worth a check for overflow in (size+npages) there.

> +
> +	return H_SUCCESS;
> +}
> +
>  /* WARNING: This will be called in real-mode on HV KVM and virtual
>   *          mode on PR KVM
>   */
>  long kvmppc_h_put_tce(struct kvm_vcpu *vcpu, unsigned long liobn,
>  		      unsigned long ioba, unsigned long tce)
>  {
> -	struct kvm *kvm = vcpu->kvm;
> -	struct kvmppc_spapr_tce_table *stt;
> +	struct kvmppc_spapr_tce_table *stt = kvmppc_find_table(vcpu, liobn);
> +	long ret = H_TOO_HARD;
> +	unsigned long idx;
> +	struct page *page;
> +	u64 *tbl;
>  
>  	/* udbg_printf("H_PUT_TCE(): liobn=0x%lx ioba=0x%lx, tce=0x%lx\n", */
>  	/* 	    liobn, ioba, tce); */
>  
> -	list_for_each_entry(stt, &kvm->arch.spapr_tce_tables, list) {
> -		if (stt->liobn == liobn) {
> -			unsigned long idx = ioba >> SPAPR_TCE_SHIFT;
> -			struct page *page;
> -			u64 *tbl;
> +	if (!stt)
> +		return ret;
>  
> -			/* udbg_printf("H_PUT_TCE: liobn 0x%lx => stt=%p  window_size=0x%x\n", */
> -			/* 	    liobn, stt, stt->window_size); */
> -			if (ioba >= stt->window_size)
> -				return H_PARAMETER;
> +	ret = kvmppc_ioba_validate(stt, ioba, 1);
> +	if (ret)
> +		return ret;
>  
> -			page = stt->pages[idx / TCES_PER_PAGE];
> -			tbl = (u64 *)page_address(page);
> +	idx = ioba >> SPAPR_TCE_SHIFT;
> +	page = stt->pages[idx / TCES_PER_PAGE];
> +	tbl = (u64 *)page_address(page);
>  
> -			/* FIXME: Need to validate the TCE itself */
> -			/* udbg_printf("tce @ %p\n", &tbl[idx % TCES_PER_PAGE]); */
> -			tbl[idx % TCES_PER_PAGE] = tce;
> -			return H_SUCCESS;
> -		}
> -	}
> +	/* FIXME: Need to validate the TCE itself */
> +	/* udbg_printf("tce @ %p\n", &tbl[idx % TCES_PER_PAGE]); */
> +	tbl[idx % TCES_PER_PAGE] = tce;
>  
> -	/* Didn't find the liobn, punt it to userspace */
> -	return H_TOO_HARD;
> +	return ret;

So, this relies on the fact that kvmppc_ioba_validate() would have
returned H_SUCCESS some distance above.  This seems rather fragile if
you insert anything else which alters ret in between.  Since this is
the success path, I think it's clearer to explicitly return H_SUCCESS.

>  }
>  EXPORT_SYMBOL_GPL(kvmppc_h_put_tce);
>  
>  long kvmppc_h_get_tce(struct kvm_vcpu *vcpu, unsigned long liobn,
>  		      unsigned long ioba)
>  {
> -	struct kvm *kvm = vcpu->kvm;
> -	struct kvmppc_spapr_tce_table *stt;
> +	struct kvmppc_spapr_tce_table *stt = kvmppc_find_table(vcpu, liobn);
> +	long ret = H_TOO_HARD;
>  
> -	list_for_each_entry(stt, &kvm->arch.spapr_tce_tables, list) {
> -		if (stt->liobn == liobn) {
> +
> +	if (stt) {
> +		ret = kvmppc_ioba_validate(stt, ioba, 1);
> +		if (!ret) {

This relies on the fact that H_SUCCESS == 0, I'm not sure if that's
something we're already doing elsewhere or not.

>  			unsigned long idx = ioba >> SPAPR_TCE_SHIFT;
> -			struct page *page;
> -			u64 *tbl;
> -
> -			if (ioba >= stt->window_size)
> -				return H_PARAMETER;
> -
> -			page = stt->pages[idx / TCES_PER_PAGE];
> -			tbl = (u64 *)page_address(page);
> +			struct page *page = stt->pages[idx / TCES_PER_PAGE];
> +			u64 *tbl = (u64 *)page_address(page);
>  
>  			vcpu->arch.gpr[4] = tbl[idx % TCES_PER_PAGE];
> -			return H_SUCCESS;
>  		}
>  	}
>  
> -	/* Didn't find the liobn, punt it to userspace */
> -	return H_TOO_HARD;
> +
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(kvmppc_h_get_tce);

-- 
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