[RFC PATCH v3 10/24] KVM: arm64: Extend reset_unknown() to handle mixed RES0/UNKNOWN registers

[Dave Martin <Dave.Martin@arm.com>]

The reset_unknown() system register helper initialises a guest
register to a distinctive junk value on vcpu reset, to help expose
and debug deficient register initialisation within the guest.

Some registers such as the SVE control register ZCR_EL1 contain a
mixture of UNKNOWN fields and RES0 bits.  For these,
reset_unknown() does not work at present, since it sets all bits to
junk values instead of just the wanted bits.

There is no need to craft another special helper just for that,
since reset_unknown() almost does the appropriate thing anyway.
This patch takes advantage of the ununused val field in struct
sys_reg_desc to specify a mask of bits that should be initialised
to zero instead of junk.

All existing users of reset_unknown() do not (and should not)
define a value for val, so they will implicitly set it to zero,
resulting in all bits being made UNKNOWN by this function: thus,
this patch makes no functional change for currently defined
registers.

Future patches will make use of non-zero val.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>

---

Changes since RFC v2:

 * Document the reset_unknown() semantics of val in the struct
   sys_reg_desc definition in sys_regs.h.
---
 arch/arm64/kvm/sys_regs.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
index cd710f8..add27cc 100644
--- a/arch/arm64/kvm/sys_regs.h
+++ b/arch/arm64/kvm/sys_regs.h
@@ -53,7 +53,12 @@ struct sys_reg_desc {
 	/* Index into sys_reg[], or 0 if we don't need to save it. */
 	int reg;
 
-	/* Value (usually reset value) */
+	/*
+	 * Value (usually reset value)
+	 * For reset_unknown, each bit set to 1 in val is treated as
+	 * RES0 in the register: the corresponding register bit is
+	 * reset to 0 instead of "unknown".
+	 */
 	u64 val;
 
 	/* Custom get/set_user functions, fallback to generic if NULL */
@@ -89,7 +94,9 @@ static inline void reset_unknown(struct kvm_vcpu *vcpu,
 {
 	BUG_ON(!r->reg);
 	BUG_ON(r->reg >= NR_SYS_REGS);
-	__vcpu_sys_reg(vcpu, r->reg) = 0x1de7ec7edbadc0deULL;
+
+	/* If non-zero, r->val specifies which register bits are RES0: */
+	__vcpu_sys_reg(vcpu, r->reg) = 0x1de7ec7edbadc0deULL & ~r->val;
 }
 
 static inline void reset_val(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
-- 
2.1.4


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