[RFC PATCH v2 19/31] KVM: arm64: Describe AT instruction emulation design

[Jintack Lim <jintack.lim@linaro.org>]

This design overview will help to digest the subsequent patches that
implement AT instruction emulation.

Signed-off-by: Jintack Lim <jintack.lim@linaro.org>
---
 arch/arm64/kvm/sys_regs.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 8d04926..d8728cc 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1621,6 +1621,72 @@ static bool access_id_aa64mmfr0_el1(struct kvm_vcpu *v,
 	{ SYS_DESC(SYS_SP_EL2), NULL, reset_special, SP_EL2, 0},
 };
 
+/*
+ * AT instruction emulation
+ *
+ * We emulate AT instructions executed in the virtual EL2.
+ * Basic strategy for the stage-1 translation emulation is to load proper
+ * context, which depends on the trapped instruction and the virtual HCR_EL2,
+ * to the EL1 virtual memory control registers and execute S1E[01] instructions
+ * in EL2. See below for more detail.
+ *
+ * For the stage-2 translation, which is necessary for S12E[01] emulation,
+ * we walk the guest hypervisor's stage-2 page table in software.
+ *
+ * The stage-1 translation emulations can be divided into two groups depending
+ * on the translation regime.
+ *
+ * 1. EL2 AT instructions: S1E2x
+ * +-----------------------------------------------------------------------+
+ * |                             |         Setting for the emulation       |
+ * | Virtual HCR_EL2.E2H on trap |-----------------------------------------+
+ * |                             | Phys EL1 regs | Phys NV, NV1 | Phys TGE |
+ * |-----------------------------------------------------------------------|
+ * |             0               |     vEL2      |    (1, 1)    |    0     |
+ * |             1               |     vEL2      |    (0, 0)    |    0     |
+ * +-----------------------------------------------------------------------+
+ *
+ * We emulate the EL2 AT instructions by loading virtual EL2 context
+ * to the EL1 virtual memory control registers and executing corresponding
+ * EL1 AT instructions.
+ *
+ * We set physical NV and NV1 bits to use EL2 page table format for non-VHE
+ * guest hypervisor (i.e. HCR_EL2.E2H == 0). As a VHE guest hypervisor uses the
+ * EL1 page table format, we don't set those bits.
+ *
+ * We should clear physical TGE bit not to use the EL2 translation regime when
+ * the host uses the VHE feature.
+ *
+ *
+ * 2. EL0/EL1 AT instructions: S1E[01]x, S12E1x
+ * +----------------------------------------------------------------------+
+ * |   Virtual HCR_EL2 on trap  |        Setting for the emulation        |
+ * |----------------------------------------------------------------------+
+ * | (vE2H, vTGE) | (vNV, vNV1) | Phys EL1 regs | Phys NV, NV1 | Phys TGE |
+ * |----------------------------------------------------------------------|
+ * |    (0, 0)*   |   (0, 0)    |      vEL1     |    (0, 0)    |    0     |
+ * |    (0, 0)    |   (1, 1)    |      vEL1     |    (1, 1)    |    0     |
+ * |    (1, 1)    |   (0, 0)    |      vEL2     |    (0, 0)    |    0     |
+ * |    (1, 1)    |   (1, 1)    |      vEL2     |    (1, 1)    |    0     |
+ * +----------------------------------------------------------------------+
+ *
+ * *For (0, 0) in the 'Virtual HCR_EL2 on trap' column, it actually means
+ *  (1, 1). Keep them (0, 0) just for the readability.
+ *
+ * We set physical EL1 virtual memory control registers depending on
+ * (vE2H, vTGE) pair. When the pair is (0, 0) where AT instructions are
+ * supposed to use EL0/EL1 translation regime, we load the EL1 registers with
+ * the virtual EL1 registers (i.e. EL1 registers from the guest hypervisor's
+ * point of view). When the pair is (1, 1), however, AT instructions are defined
+ * to apply EL2 translation regime. To emulate this behavior, we load the EL1
+ * registers with the virtual EL2 context. (i.e the shadow registers)
+ *
+ * We respect the virtual NV and NV1 bit for the emulation. When those bits are
+ * set, it means that a guest hypervisor would like to use EL2 page table format
+ * for the EL1 translation regime. We emulate this by setting the physical
+ * NV and NV1 bits.
+ */
+
 #define SYS_INSN_TO_DESC(insn, access_fn, forward_fn)	\
 	{ SYS_DESC((insn)), (access_fn), NULL, 0, 0, NULL, NULL, (forward_fn) }
 static struct sys_reg_desc sys_insn_descs[] = {
-- 
1.9.1