linux-kernel - Re: [PATCH v6 07/11] powerpc/kvm/e500: Use functions to track lockless pgtbl walks

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Date:   Thu, 6 Feb 2020 07:18:02 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Leonardo Bras <leonardo@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Steven Price <steven.price@....com>,
        Robin Murphy <robin.murphy@....com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        Reza Arbab <arbab@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Allison Randal <allison@...utok.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michal Suchanek <msuchanek@...e.de>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v6 07/11] powerpc/kvm/e500: Use functions to track
 lockless pgtbl walks



Le 06/02/2020 à 04:08, Leonardo Bras a écrit :
> Applies the new functions used for tracking lockless pgtable walks on
> kvmppc_e500_shadow_map().
> 
> Fixes the place where local_irq_restore() is called: previously, if ptep
> was NULL, local_irq_restore() would never be called.
> 
> local_irq_{save,restore} is already inside {begin,end}_lockless_pgtbl_walk,
> so there is no need to repeat it here.
> 
> Variable that saves the	irq mask was renamed from flags to irq_mask so it
> doesn't lose meaning now it's not directly passed to local_irq_* functions.
> 
> Signed-off-by: Leonardo Bras <leonardo@...ux.ibm.com>
> ---
>   arch/powerpc/kvm/e500_mmu_host.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/e500_mmu_host.c b/arch/powerpc/kvm/e500_mmu_host.c
> index 425d13806645..3dcf11f77256 100644
> --- a/arch/powerpc/kvm/e500_mmu_host.c
> +++ b/arch/powerpc/kvm/e500_mmu_host.c
> @@ -336,7 +336,7 @@ static inline int kvmppc_e500_shadow_map(struct kvmppc_vcpu_e500 *vcpu_e500,
>   	pte_t *ptep;
>   	unsigned int wimg = 0;
>   	pgd_t *pgdir;
> -	unsigned long flags;
> +	unsigned long irq_mask;
>   
>   	/* used to check for invalidations in progress */
>   	mmu_seq = kvm->mmu_notifier_seq;
> @@ -473,7 +473,7 @@ static inline int kvmppc_e500_shadow_map(struct kvmppc_vcpu_e500 *vcpu_e500,
>   	 * We are holding kvm->mmu_lock so a notifier invalidate
>   	 * can't run hence pfn won't change.
>   	 */
> -	local_irq_save(flags);
> +	irq_mask = begin_lockless_pgtbl_walk();
>   	ptep = find_linux_pte(pgdir, hva, NULL, NULL);
>   	if (ptep) {
>   		pte_t pte = READ_ONCE(*ptep);
> @@ -481,15 +481,16 @@ static inline int kvmppc_e500_shadow_map(struct kvmppc_vcpu_e500 *vcpu_e500,
>   		if (pte_present(pte)) {
>   			wimg = (pte_val(pte) >> PTE_WIMGE_SHIFT) &
>   				MAS2_WIMGE_MASK;
> -			local_irq_restore(flags);
>   		} else {
> -			local_irq_restore(flags);
> +			end_lockless_pgtbl_walk(irq_mask);
>   			pr_err_ratelimited("%s: pte not present: gfn %lx,pfn %lx\n",
>   					   __func__, (long)gfn, pfn);
>   			ret = -EINVAL;
>   			goto out;
>   		}
>   	}
> +	end_lockless_pgtbl_walk(irq_mask);
> +

I don't really like unbalanced begin/end.

Something like the following would be cleaner:


begin_lockless_pgtbl_walk()
ptep = find()
if (ptep) {
	pte = READ_ONCE()
	if (pte_present(pte))
		wing=
	else
		ret = -EINVAL;
}
end_lockless_pgtbl_walk()

if (ret) {
	pr_err_rate...()
	goto out;
}



>   	kvmppc_e500_ref_setup(ref, gtlbe, pfn, wimg);
>   
>   	kvmppc_e500_setup_stlbe(&vcpu_e500->vcpu, gtlbe, tsize,
> 

Christophe