Hi Andrew,

As per current implementation, in “el1_sysregs_context_restore” routine do below things:

1. TCR_EL1.EPD0 = 1       2. TCR_EL1.EPD1 = 1       3. SCTR_EL1.M = 0 4. Isb Code snippet: mrs x9, tcr_el1 orr x9, x9, #TCR_EPD0_BIT orr x9, x9, #TCR_EPD1_BIT msr tcr_el1, x9 mrs x9, sctlr_el1 bic x9, x9, #SCTLR_M_BIT msr sctlr_el1, x9 isb This is to avoid PTW through while updating system registers at step 5  5. Restore all system registers for El1 except SCTLR_EL1 and TCR_EL1       6. isb()       7. restore SCTLR_EL1 and TCR_EL1 Code Snippet: ldr x9, [x0, #CTX_SCTLR_EL1] -> saved value from "el2_sysregs_context_save" msr sctlr_el1, x9 ldr x9, [x0, #CTX_TCR_EL1] msr tcr_el1, x9

As per above steps. SCTLR_EL1 get restored back with actual settings at step 7. Similar flow is present for “el2_sysregs_context_restore” to restore SCTLR_EL1 register.

In conclusion, this routine temporarily clear M bit of SCTLR_EL1 to avoid speculation but restored it back to its original setting while leaving back to its caller. Please let us know whether this align with KVM workaround for speculative AT erratum.

Thanks Manish Badarkhe

On 01/07/2020, 17:02, "TF-A on behalf of Joanna Farley via TF-A" <tf-a-bounces@lists.trustedfirmware.org on behalf of tf-a@lists.trustedfirmware.org> wrote:

Thanks Andrew for the notification. We will look into it and get back to this thread.

Thanks

Joanna

On 01/07/2020, 10:16, "TF-A on behalf of Andrew Scull via TF-A" <tf-a-bounces@lists.trustedfirmware.org on behalf of tf-a@lists.trustedfirmware.org> wrote:

Whilst looking at the speculative AT workaround in KVM, I compared it against the workaround in TF-A and noticed an inconsistency whereby TF-A **breaks** KVM's workaround.

In `el1_sysregs_context_restore`, the M bit of SCTRL_EL1 is cleared however Linux requires this to be set for its workaround to be correct. If an exception is taken to EL3 partway through a VM context switch, e.g. a secure interrupt, causing a switch to the secure world, TF-A will reintroduce the possibility of TLB corruption.

The above explains how it is broken for Linux's chosen workaround however TF-A will also have to be compatible with whatever workaround the EL2 software is using.

Starting this thread with the issue identified and we can add more details as needed.

Thanks Andrew -- TF-A mailing list TF-A@lists.trustedfirmware.org https://lists.trustedfirmware.org/mailman/listinfo/tf-a

-- TF-A mailing list TF-A@lists.trustedfirmware.org https://lists.trustedfirmware.org/mailman/listinfo/tf-a