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Message-ID: <865xf09t3i.wl-maz@kernel.org>
Date: Wed, 06 Aug 2025 19:56:49 +0100
From: Marc Zyngier <maz@...nel.org>
To: Volodymyr Babchuk <Volodymyr_Babchuk@...m.com>
Cc: "linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"kvmarm@...ts.linux.dev"
<kvmarm@...ts.linux.dev>,
"linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
Joey Gouly
<joey.gouly@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Zenghui Yu
<yuzenghui@...wei.com>,
Catalin Marinas <catalin.marinas@....com>,
Will
Deacon <will@...nel.org>
Subject: Re: [PATCH v1 2/2] KVM: arm64: nv: update CPU register PAR_EL1 after 'at s*'
On Wed, 06 Aug 2025 15:17:55 +0100,
Volodymyr Babchuk <Volodymyr_Babchuk@...m.com> wrote:
>
> Previously this code update only vCPU's in-memory value, which is good,
> but not enough, as there will be no context switch after exiting
> exception handler, so in-memory value will not get into actual
> register.
>
> It worked good enough for VHE guests because KVM code tried fast path,
> which of course updated real PAR_EL1.
Nothing to do with VHE, I'm afraid. We can take the slow path for any
odd reason, even on VHE. This is more of a structural problem, see
below.
>
> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@...m.com>
> ---
> arch/arm64/kvm/sys_regs.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 7b8a0a6f26468..ab2b5e261d312 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -3463,6 +3463,9 @@ static bool handle_at_s1e2(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
>
> __kvm_at_s1e2(vcpu, op, p->regval);
>
> + /* No context switch happened, so we need to update PAR_EL1 manually */
> + write_sysreg(vcpu_read_sys_reg(vcpu, PAR_EL1), par_el1);
> +
This looks like the wrong fix, as it papers over another issue.
The core problem is vcpu_write_sys_reg() (resp. read) does the wrong
thing with registers such as PAR_EL1, which are not translated between
EL1 and EL2, and therefore are always live, no matter what.
Can you please try the hack below? I don't like it, but at least it
shows where the actual problem lies.
Thanks,
M.
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index ad25484772574..167f0d411a708 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -95,7 +95,13 @@ static bool write_to_read_only(struct kvm_vcpu *vcpu,
return true; \
}
-static bool get_el2_to_el1_mapping(unsigned int reg,
+#define COMMON_SYSREG(r) \
+ case r: { \
+ *el1r = __INVALID_SYSREG__; \
+ return is_hyp_ctxt(vcpu); \
+ }
+
+static bool get_el2_to_el1_mapping(const struct kvm_vcpu *vcpu, unsigned int reg,
unsigned int *el1r, u64 (**xlate)(u64))
{
switch (reg) {
@@ -119,6 +125,7 @@ static bool get_el2_to_el1_mapping(unsigned int reg,
PURE_EL2_SYSREG( HAFGRTR_EL2 );
PURE_EL2_SYSREG( CNTVOFF_EL2 );
PURE_EL2_SYSREG( CNTHCTL_EL2 );
+ COMMON_SYSREG( PAR_EL1 );
MAPPED_EL2_SYSREG(SCTLR_EL2, SCTLR_EL1,
translate_sctlr_el2_to_sctlr_el1 );
MAPPED_EL2_SYSREG(CPTR_EL2, CPACR_EL1,
@@ -158,7 +165,7 @@ u64 vcpu_read_sys_reg(const struct kvm_vcpu *vcpu, int reg)
if (!vcpu_get_flag(vcpu, SYSREGS_ON_CPU))
goto memory_read;
- if (unlikely(get_el2_to_el1_mapping(reg, &el1r, &xlate))) {
+ if (unlikely(get_el2_to_el1_mapping(vcpu, reg, &el1r, &xlate))) {
if (!is_hyp_ctxt(vcpu))
goto memory_read;
@@ -219,7 +226,7 @@ void vcpu_write_sys_reg(struct kvm_vcpu *vcpu, u64 val, int reg)
if (!vcpu_get_flag(vcpu, SYSREGS_ON_CPU))
goto memory_write;
- if (unlikely(get_el2_to_el1_mapping(reg, &el1r, &xlate))) {
+ if (unlikely(get_el2_to_el1_mapping(vcpu, reg, &el1r, &xlate))) {
if (!is_hyp_ctxt(vcpu))
goto memory_write;
--
Without deviation from the norm, progress is not possible.
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