On Wed, Apr 17, 2019 at 03:09:02PM +0530, Amit Daniel Kachhap wrote: > Hi, > > On 4/16/19 10:02 PM, Dave Martin wrote: > >On Fri, Apr 12, 2019 at 08:50:35AM +0530, Amit Daniel Kachhap wrote: > >>This patch advertises the capability of two cpu feature called address > >>pointer authentication and generic pointer authentication. These > >>capabilities depend upon system support for pointer authentication and > >>VHE mode. > >> > >>The current arm64 KVM partially implements pointer authentication and > >>support of address/generic authentication are tied together. However, > >>separate ABI requirements for both of them is added so that any future > >>isolated implementation will not require any ABI changes. > >> > >>Signed-off-by: Amit Daniel Kachhap <amit.kachhap@xxxxxxx> > >>Cc: Mark Rutland <mark.rutland@xxxxxxx> > >>Cc: Marc Zyngier <marc.zyngier@xxxxxxx> > >>Cc: Christoffer Dall <christoffer.dall@xxxxxxx> > >>Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx > >>--- > >>Changes since v8: > >>* Keep the capability check same for the 2 vcpu ptrauth features. [Dave Martin] > >> > >> Documentation/virtual/kvm/api.txt | 2 ++ > >> arch/arm64/kvm/reset.c | 5 +++++ > >> include/uapi/linux/kvm.h | 2 ++ > >> 3 files changed, 9 insertions(+) > >> > >>diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt > >>index 9d202f4..56021d0 100644 > >>--- a/Documentation/virtual/kvm/api.txt > >>+++ b/Documentation/virtual/kvm/api.txt > >>@@ -2756,9 +2756,11 @@ Possible features: > >> - KVM_ARM_VCPU_PTRAUTH_ADDRESS: Enables Address Pointer authentication > >> for the CPU and supported only on arm64 architecture. > >> Must be requested if KVM_ARM_VCPU_PTRAUTH_GENERIC is also requested. > >>+ Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS. > > > >What if KVM_CAP_ARM_PTRAUTH_ADDRESS is absent and > >KVM_ARM_VCPU_PTRAUTH_GENERIC is requested? By these rules, we have a > >contradiction: userspace both must request and must not request > >KVM_ARM_VCPU_PTRAUTH_ADDRESS. > > > >We could qualify as follows: > > > > Depends on KVM_CAP_ARM_PTRAUTH_ADDRESS. > > Must be requested if KVM_CAP_ARM_PTRAUTH_ADDRESS is present and > > KVM_ARM_VCPU_PTRAUTH_GENERIC is also requested. > ok agree. This makes it clear. [*] > >> - KVM_ARM_VCPU_PTRAUTH_GENERIC: Enables Generic Pointer authentication > >> for the CPU and supported only on arm64 architecture. > >> Must be requested if KVM_ARM_VCPU_PTRAUTH_ADDRESS is also requested. > >>+ Depends on KVM_CAP_ARM_PTRAUTH_GENERIC. > > > >Similarly. > > > >Or, we go back to having a single cap and a single feature, and add > >more caps/features later on if we decide it's possible to support > >address/generic auth separately later on. > > > >Otherwise, we end up with complex rules that can't be tested. This is a > >high price to pay for forwards compatibility: userspace's conformance to > >the rules can't be fully tested, so there's a fair chance it won't work > >properly anyway when hardware/KVM with just one auth type appears. > > > >[...] > > > >Thoughts? > I agree that single cpufeature/capability is a simple solution to implement. > The bifurcation of feature was done to reflect the different ID register > split up. > > But the h/w implementation provides a same EL2 exception trap for both the > features and hence current implementation ties both of the features > together. I guess in future if this is limitation goes away then one auth > type is possible. Here I am not sure if the future h/w will retain this > merged exception trap and add 2 new separate exception trap in addition to > it. > > I guess it will be probably simple split-up of this merged exception trap. > In this case there won't be any ABI change required as per current > implementation. OK, I'm not opposed to keeping the ABI as-is, with the above clarification [*] spelled out appropriately for both cases. Alternatively, or in addition, we could say something like: "If KVM_CAP_ARM_PTRAUTH_ADDRESS and KVM_CAP_ARM_PTRAUTH_GENERIC are both present, then both KVM_ARM_VCPU_PTRAUTH_ADDRESS and KVM_ARM_VCPU_PTRAUTH_GENERIC must be requested or neither must be requested." Cheers ---Dave _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
