Messages in this thread | | | Subject | Re: [PATCH v9 4/5] KVM: arm64: Add capability to advertise ptrauth for guest | From | Amit Daniel Kachhap <> | Date | Wed, 17 Apr 2019 15:09:02 +0530 |
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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@arm.com> >> Cc: Mark Rutland <mark.rutland@arm.com> >> Cc: Marc Zyngier <marc.zyngier@arm.com> >> Cc: Christoffer Dall <christoffer.dall@arm.com> >> Cc: kvmarm@lists.cs.columbia.edu >> --- >> 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.
Thanks, Amit Daniel
> > Cheers > ---Dave >
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