Re: [Qemu-devel] [PATCH 06/12] target/ppc: Fix ordering of hash MMU accesses

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

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On Fri, Feb 15, 2019 at 06:00:23PM +0100, Cédric Le Goater wrote:
> From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> 
> With mttcg, we can have MMU lookups happening at the same time
> as the guest modifying the page tables.
> 
> Since the HPTEs of the hash table MMU contains two words (or
> double worlds on 64-bit), we need to make sure we read them
> in the right order, with the correct memory barrier.
> 
> Additionally, when using emulated SPAPR mode, the hypercalls
> writing to the hash table must also perform the udpates in
> the right order.
> 
> Note: This part is still not entirely correct
> 
> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Signed-off-by: Cédric Le Goater <clg@kaod.org>

Yeah, this stuff was written under the assumption it was protected by
the BQL, which is getting less true all the time.

Applied.

> ---
>  hw/ppc/spapr.c          | 21 +++++++++++++++++++--
>  target/ppc/mmu-hash32.c |  6 ++++++
>  target/ppc/mmu-hash64.c |  6 ++++++
>  3 files changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 60572eb59275..1afe31ee6163 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -1507,8 +1507,25 @@ static void spapr_store_hpte(PPCVirtualHypervisor *vhyp, hwaddr ptex,
>      if (!spapr->htab) {
>          kvmppc_write_hpte(ptex, pte0, pte1);
>      } else {
> -        stq_p(spapr->htab + offset, pte0);
> -        stq_p(spapr->htab + offset + HASH_PTE_SIZE_64 / 2, pte1);
> +        if (pte0 & HPTE64_V_VALID) {
> +            stq_p(spapr->htab + offset + HASH_PTE_SIZE_64 / 2, pte1);
> +            /*
> +             * When setting valid, we write PTE1 first. This ensures
> +             * proper synchronization with the reading code in
> +             * ppc_hash64_pteg_search()
> +             */
> +            smp_wmb();
> +            stq_p(spapr->htab + offset, pte0);
> +        } else {
> +            stq_p(spapr->htab + offset, pte0);
> +            /*
> +             * When clearing it we set PTE0 first. This ensures proper
> +             * synchronization with the reading code in
> +             * ppc_hash64_pteg_search()
> +             */
> +            smp_wmb();
> +            stq_p(spapr->htab + offset + HASH_PTE_SIZE_64 / 2, pte1);
> +        }
>      }
>  }
>  
> diff --git a/target/ppc/mmu-hash32.c b/target/ppc/mmu-hash32.c
> index 03ae3c127985..e8562a7c8780 100644
> --- a/target/ppc/mmu-hash32.c
> +++ b/target/ppc/mmu-hash32.c
> @@ -319,6 +319,12 @@ static hwaddr ppc_hash32_pteg_search(PowerPCCPU *cpu, hwaddr pteg_off,
>  
>      for (i = 0; i < HPTES_PER_GROUP; i++) {
>          pte0 = ppc_hash32_load_hpte0(cpu, pte_offset);
> +        /*
> +         * pte0 contains the valid bit and must be read before pte1,
> +         * otherwise we might see an old pte1 with a new valid bit and
> +         * thus an inconsistent hpte value
> +         */
> +        smp_rmb();
>          pte1 = ppc_hash32_load_hpte1(cpu, pte_offset);
>  
>          if ((pte0 & HPTE32_V_VALID)
> diff --git a/target/ppc/mmu-hash64.c b/target/ppc/mmu-hash64.c
> index f6c822ef917b..b3c4d33faa55 100644
> --- a/target/ppc/mmu-hash64.c
> +++ b/target/ppc/mmu-hash64.c
> @@ -506,6 +506,12 @@ static hwaddr ppc_hash64_pteg_search(PowerPCCPU *cpu, hwaddr hash,
>      }
>      for (i = 0; i < HPTES_PER_GROUP; i++) {
>          pte0 = ppc_hash64_hpte0(cpu, pteg, i);
> +        /*
> +         * pte0 contains the valid bit and must be read before pte1,
> +         * otherwise we might see an old pte1 with a new valid bit and
> +         * thus an inconsistent hpte value
> +         */
> +        smp_rmb();
>          pte1 = ppc_hash64_hpte1(cpu, pteg, i);
>  
>          /* This compares V, B, H (secondary) and the AVPN */

-- 
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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