From 75d08a407240754e2ca35caa5852d1d33763856c Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Tue, 26 Apr 2022 17:04:21 +0100 Subject: [PATCH] target/arm: Advertise support for FEAT_BBM level 2 The description in the Arm ARM of the requirements of FEAT_BBM is admirably clear on the guarantees it provides software, but slightly more obscure on what that means for implementations. The description of the equivalent SMMU feature in the SMMU specification (IHI0070D.b section 3.21.1) is perhaps a bit more detailed and includes some example valid implementation choices. (The SMMU version of this feature is slightly tighter than the CPU version: the CPU is permitted to raise TLB Conflict aborts in some situations that the SMMU may not. This doesn't matter for QEMU because we don't want to do TLB Conflict aborts anyway.) The informal summary of FEAT_BBM is that it is about permitting an OS to switch a range of memory between "covered by a huge page" and "covered by a sequence of normal pages" without having to engage in the 'break-before-make' dance that has traditionally been necessary. The 'break-before-make' sequence is: * replace the old translation table entry with an invalid entry * execute a DSB insn * execute a broadcast TLB invalidate insn * execute a DSB insn * write the new translation table entry * execute a DSB insn The point of this is to ensure that no TLB can simultaneously contain TLB entries for the old and the new entry, which would traditionally be UNPREDICTABLE (allowing the CPU to generate a TLB Conflict fault or to use a random mishmash of values from the old and the new entry). FEAT_BBM level 2 says "for the specific case where the only thing that changed is the size of the block, the TLB is guaranteed not to do weird things even if there are multiple entries for an address", which means that software can now do: * replace old translation table entry with new entry * DSB * broadcast TLB invalidate * DSB As the SMMU spec notes, valid ways to do this include: * if there are multiple entries in the TLB for an address, choose one of them and use it, ignoring the others * if there are multiple entries in the TLB for an address, throw them all out and do a page table walk to get a new one QEMU's page table walk implementation for Arm CPUs already meets the requirements for FEAT_BBM level 2. When we cache an entry in our TCG TLB, we do so only for the specific (non-huge) page that the address is in, and there is no way for the TLB data structure to ever have more than one TLB entry for that page. (We handle huge pages only in that we track what part of the address space is covered by huge pages so that a TLB invalidate operation for an address in a huge page results in an invalidation of the whole TLB.) We ignore the Contiguous bit in page table entries, so we don't have to do anything for the parts of FEAT_BBM that deal with changis to the Contiguous bit. FEAT_BBM level 2 also requires that the nT bit in block descriptors must be ignored; since commit 39a1fd25287f5dece5 we do this. It's therefore safe for QEMU to advertise FEAT_BBM level 2 by setting ID_AA64MMFR2_EL1.BBM to 2. Signed-off-by: Peter Maydell Reviewed-by: Richard Henderson Message-id: 20220426160422.2353158-3-peter.maydell@linaro.org --- docs/system/arm/emulation.rst | 1 + target/arm/cpu64.c | 1 + 2 files changed, 2 insertions(+) diff --git a/docs/system/arm/emulation.rst b/docs/system/arm/emulation.rst index 6ed2417f6f..c3bd0676a8 100644 --- a/docs/system/arm/emulation.rst +++ b/docs/system/arm/emulation.rst @@ -9,6 +9,7 @@ the following architecture extensions: - FEAT_AA32HPD (AArch32 hierarchical permission disables) - FEAT_AA32I8MM (AArch32 Int8 matrix multiplication instructions) - FEAT_AES (AESD and AESE instructions) +- FEAT_BBM at level 2 (Translation table break-before-make levels) - FEAT_BF16 (AArch64 BFloat16 instructions) - FEAT_BTI (Branch Target Identification) - FEAT_DIT (Data Independent Timing instructions) diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c index ec2d159163..2974cbc0d3 100644 --- a/target/arm/cpu64.c +++ b/target/arm/cpu64.c @@ -840,6 +840,7 @@ static void aarch64_max_initfn(Object *obj) t = FIELD_DP64(t, ID_AA64MMFR2, ST, 1); /* TTST */ t = FIELD_DP64(t, ID_AA64MMFR2, VARANGE, 1); /* FEAT_LVA */ t = FIELD_DP64(t, ID_AA64MMFR2, TTL, 1); /* FEAT_TTL */ + t = FIELD_DP64(t, ID_AA64MMFR2, BBM, 2); /* FEAT_BBM at level 2 */ cpu->isar.id_aa64mmfr2 = t; t = cpu->isar.id_aa64zfr0;