On Thu, Feb 21, 2019 at 12:29:42PM +0000, Mark Rutland wrote: > On Tue, Feb 19, 2019 at 02:54:28PM +0530, Amit Daniel Kachhap wrote: > > From: Mark Rutland <mark.rutland at arm.com> > > > > When pointer authentication is supported, a guest may wish to use it. > > This patch adds the necessary KVM infrastructure for this to work, with > > a semi-lazy context switch of the pointer auth state. > > > > Pointer authentication feature is only enabled when VHE is built > > in the kernel and present into CPU implementation so only VHE code > > paths are modified. > > Nit: s/into/in the/ > > > > > When we schedule a vcpu, we disable guest usage of pointer > > authentication instructions and accesses to the keys. While these are > > disabled, we avoid context-switching the keys. When we trap the guest > > trying to use pointer authentication functionality, we change to eagerly > > context-switching the keys, and enable the feature. The next time the > > vcpu is scheduled out/in, we start again. However the host key registers > > are saved in vcpu load stage as they remain constant for each vcpu > > schedule. > > > > Pointer authentication consists of address authentication and generic > > authentication, and CPUs in a system might have varied support for > > either. Where support for either feature is not uniform, it is hidden > > from guests via ID register emulation, as a result of the cpufeature > > framework in the host. > > > > Unfortunately, address authentication and generic authentication cannot > > be trapped separately, as the architecture provides a single EL2 trap > > covering both. If we wish to expose one without the other, we cannot > > prevent a (badly-written) guest from intermittently using a feature > > which is not uniformly supported (when scheduled on a physical CPU which > > supports the relevant feature). Hence, this patch expects both type of > > authentication to be present in a cpu. > > > > Signed-off-by: Mark Rutland <mark.rutland at arm.com> > > [Only VHE, key switch from from assembly, kvm_supports_ptrauth > > checks, save host key in vcpu_load] > > Hmm, why do we need to do the key switch in assembly, given it's not > used in-kernel right now? > > Is that in preparation for in-kernel pointer auth usage? If so, please > call that out in the commit message. [...] > Huh, so we're actually doing the switch in C code... > > > # KVM code is run at a different exception code with a different map, so > > # compiler instrumentation that inserts callbacks or checks into the code may > > diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S > > index 675fdc1..b78cc15 100644 > > --- a/arch/arm64/kvm/hyp/entry.S > > +++ b/arch/arm64/kvm/hyp/entry.S > > @@ -64,6 +64,12 @@ ENTRY(__guest_enter) > > > > add x18, x0, #VCPU_CONTEXT > > > > +#ifdef CONFIG_ARM64_PTR_AUTH > > + // Prepare parameter for __ptrauth_switch_to_guest(vcpu, host, guest). > > + mov x2, x18 > > + bl __ptrauth_switch_to_guest > > +#endif > > ... and conditionally *calling* that switch code from assembly ... > > > + > > // Restore guest regs x0-x17 > > ldp x0, x1, [x18, #CPU_XREG_OFFSET(0)] > > ldp x2, x3, [x18, #CPU_XREG_OFFSET(2)] > > @@ -118,6 +124,17 @@ ENTRY(__guest_exit) > > > > get_host_ctxt x2, x3 > > > > +#ifdef CONFIG_ARM64_PTR_AUTH > > + // Prepare parameter for __ptrauth_switch_to_host(vcpu, guest, host). > > + // Save x0, x2 which are used later in callee saved registers. > > + mov x19, x0 > > + mov x20, x2 > > + sub x0, x1, #VCPU_CONTEXT > > + ldr x29, [x2, #CPU_XREG_OFFSET(29)] > > + bl __ptrauth_switch_to_host > > + mov x0, x19 > > + mov x2, x20 > > +#endif > > ... which adds a load of boilerplate for no immediate gain. > > Do we really need to do this in assembly today? If we will need to move this to assembly when we add in-kernel ptrauth support, it may be best to have it in assembly from the start, to reduce unnecessary churn. But having a mix of C and assembly is likely to make things more complicated: we should go with one or the other IMHO. Cheers ---Dave
