Re: [PATCH v2 1/2] KVM: arm64: Allow userspace to change ID_AA64PFR1_EL1

From: Shaoqin Huang
Date: Fri Jun 21 2024 - 02:20:41 EST


Hi Marc,

On 6/18/24 15:39, Marc Zyngier wrote:
On Tue, 18 Jun 2024 07:38:06 +0100,
Shaoqin Huang <shahuang@xxxxxxxxxx> wrote:

Allow userspace to change the guest-visible value of the register with
some severe limitation:

- No changes to features not virtualized by KVM (MPAM_frac, RAS_frac)
---
arch/arm64/kvm/sys_regs.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 22b45a15d068..bead81867bce 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2306,7 +2306,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
ID_AA64PFR0_EL1_GIC |
ID_AA64PFR0_EL1_AdvSIMD |
ID_AA64PFR0_EL1_FP), },
- ID_SANITISED(ID_AA64PFR1_EL1),
+ ID_WRITABLE(ID_AA64PFR1_EL1, ~(ID_AA64PFR1_EL1_RAS_frac |
+ ID_AA64PFR1_EL1_MPAM_frac)),
ID_UNALLOCATED(4,2),
ID_UNALLOCATED(4,3),
ID_WRITABLE(ID_AA64ZFR0_EL1, ~ID_AA64ZFR0_EL1_RES0),

This isn't a valid patch.

Furthermore, how about all the other features that may or may not be
currently handled by KVM? Please see [1] and make sure that all
existing fields have a known behaviour (a combination of masked,
preserved, capped, writable or read-only).

I can at least see problems with MTE_frac and MTEX, plus all the other
things that KVM doesn't know how to save/restore (THE, GCS, NMI...).

What I asked you to handle the whole register, I really meant it.

I currently only found the BT and SSBS fields can be written without any unknown behavior.

All other fields in the ID_AA64PFR1_EL1 are either not supported by KVM or the field involved with other register and KVM don't know how to handle them.

I'm not sure if this is right. So I want to ask about your opinion about how to allow each of the field to be writable in the ID_AA64PFR1_EL1 register?

Thanks,
Shaoqin


M.

[1] https://developer.arm.com/documentation/ddi0601/2024-03/AArch64-Registers/ID-AA64PFR1-EL1--AArch64-Processor-Feature-Register-1?lang=en


--
Shaoqin