These patches add support to KVM to enable MTE within a guest. It is based on Catalin's v4 MTE user space series[1].
[1] http://lkml.kernel.org/r/20200515171612.1020-1-catalin.marinas%40arm.com Posting as an RFC as I'd like feedback on the approach taken. First a little background on how MTE fits within the architecture: The stage 2 page tables have limited scope for controlling the availability of MTE. If a page is mapped as Normal and cached in stage 2 then it's the stage 1 tables that get to choose whether the memory is tagged or not. So the only way of forbidding tags on a page from the hypervisor is to change the cacheability (or make it device memory) which would cause other problems. Note this restriction fits the intention that a system should have all (general purpose) memory supporting tags if it support MTE, so it's not too surprising. However, the upshot of this is that to enable MTE within a guest all pages of memory mapped into the guest as normal cached pages in stage 2 *must* support MTE (i.e. we must ensure the tags are appropriately sanitised and save/restore the tags during swap etc). My current approach is that KVM transparently upgrades any pages provided by the VMM to be tag-enabled when they are faulted in (i.e. sets the PG_mte_tagged flag on the page) which has the benefit of requiring fewer changes in the VMM. However, save/restore of the VM state still requires the VMM to have a PROT_MTE enabled mapping so that it can access the tag values. A VMM which 'forgets' to enable PROT_MTE would lose the tag values when saving/restoring (tags are RAZ/WI when PROT_MTE isn't set). An alternative approach would be to enforce the VMM provides PROT_MTE memory in the first place. This seems appealing to prevent the above potentially unexpected gotchas with save/restore, however this would also extend to memory that you might not expect to have PROT_MTE (e.g. a shared frame buffer for an emulated graphics card). Comments on the approach (or ideas for alternative approaches) are very welcome. Steven Price (2): arm64: kvm: Save/restore MTE registers arm64: kvm: Introduce MTE VCPU feature arch/arm64/include/asm/kvm_emulate.h | 3 +++ arch/arm64/include/asm/kvm_host.h | 9 ++++++++- arch/arm64/include/uapi/asm/kvm.h | 1 + arch/arm64/kvm/hyp/sysreg-sr.c | 12 ++++++++++++ arch/arm64/kvm/reset.c | 8 ++++++++ arch/arm64/kvm/sys_regs.c | 8 ++++++++ virt/kvm/arm/mmu.c | 11 +++++++++++ 7 files changed, 51 insertions(+), 1 deletion(-) -- 2.20.1
