niansa-misc/linux
1
0
Fork 0
mirror of synced 2025-03-06 20:59:54 +01:00
Code Issues Projects Releases Packages Wiki Activity

KVM: arm64: Eagerly switch ZCR_EL{1,2}

Browse source 
In non-protected KVM modes, while the guest FPSIMD/SVE/SME state is live on the
CPU, the host's active SVE VL may differ from the guest's maximum SVE VL:

* For VHE hosts, when a VM uses NV, ZCR_EL2 contains a value constrained
  by the guest hypervisor, which may be less than or equal to that
  guest's maximum VL.

  Note: in this case the value of ZCR_EL1 is immaterial due to E2H.

* For nVHE/hVHE hosts, ZCR_EL1 contains a value written by the guest,
  which may be less than or greater than the guest's maximum VL.

  Note: in this case hyp code traps host SVE usage and lazily restores
  ZCR_EL2 to the host's maximum VL, which may be greater than the
  guest's maximum VL.

This can be the case between exiting a guest and kvm_arch_vcpu_put_fp().
If a softirq is taken during this period and the softirq handler tries
to use kernel-mode NEON, then the kernel will fail to save the guest's
FPSIMD/SVE state, and will pend a SIGKILL for the current thread.

This happens because kvm_arch_vcpu_ctxsync_fp() binds the guest's live
FPSIMD/SVE state with the guest's maximum SVE VL, and
fpsimd_save_user_state() verifies that the live SVE VL is as expected
before attempting to save the register state:

| if (WARN_ON(sve_get_vl() != vl)) {
|         force_signal_inject(SIGKILL, SI_KERNEL, 0, 0);
|         return;
| }

Fix this and make this a bit easier to reason about by always eagerly
switching ZCR_EL{1,2} at hyp during guest<->host transitions. With this
happening, there's no need to trap host SVE usage, and the nVHE/nVHE
__deactivate_cptr_traps() logic can be simplified to enable host access
to all present FPSIMD/SVE/SME features.

In protected nVHE/hVHE modes, the host's state is always saved/restored
by hyp, and the guest's state is saved prior to exit to the host, so
from the host's PoV the guest never has live FPSIMD/SVE/SME state, and
the host's ZCR_EL1 is never clobbered by hyp.

Fixes: 8c8010d69c ("KVM: arm64: Save/restore SVE state for nVHE")
Fixes: 2e3cf82063 ("KVM: arm64: nv: Ensure correct VL is loaded before saving SVE state")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250210195226.1215254-9-mark.rutland@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
This commit is contained in:
Mark Rutland 2025-02-10 19:52:26 +00:00 • committed by Marc Zyngier
parent f9dd00de1e
commit 59419f1004
6 changed files with 76 additions and 41 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

30
arch/arm64/kvm/fpsimd.c
View file

@ -136,36 +136,6 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
local_irq_save(flags);
if (guest_owns_fp_regs()) {
if (vcpu_has_sve(vcpu)) {
u64 zcr = read_sysreg_el1(SYS_ZCR);
/*
* If the vCPU is in the hyp context then ZCR_EL1 is
* loaded with its vEL2 counterpart.
*/
__vcpu_sys_reg(vcpu, vcpu_sve_zcr_elx(vcpu)) = zcr;
/*
* Restore the VL that was saved when bound to the CPU,
* which is the maximum VL for the guest. Because the
* layout of the data when saving the sve state depends
* on the VL, we need to use a consistent (i.e., the
* maximum) VL.
* Note that this means that at guest exit ZCR_EL1 is
* not necessarily the same as on guest entry.
*
* ZCR_EL2 holds the guest hypervisor's VL when running
* a nested guest, which could be smaller than the
* max for the vCPU. Similar to above, we first need to
* switch to a VL consistent with the layout of the
* vCPU's SVE state. KVM support for NV implies VHE, so
* using the ZCR_EL1 alias is safe.
*/
if (!has_vhe() || (vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu)))
sve_cond_update_zcr_vq(vcpu_sve_max_vq(vcpu) - 1,
SYS_ZCR_EL1);
}
/*
* Flush (save and invalidate) the fpsimd/sve state so that if
* the host tries to use fpsimd/sve, it's not using stale data

5
arch/arm64/kvm/hyp/entry.S
View file

@ -44,6 +44,11 @@ alternative_if ARM64_HAS_RAS_EXTN
alternative_else_nop_endif
mrs x1, isr_el1
cbz x1, 1f
// Ensure that __guest_enter() always provides a context
// synchronization event so that callers don't need ISBs for anything
// that would usually be synchonized by the ERET.
isb
mov x0, #ARM_EXCEPTION_IRQ
ret

59
arch/arm64/kvm/hyp/include/hyp/switch.h
View file

@ -375,6 +375,65 @@ static inline void __hyp_sve_save_host(void)
true);
}
static inline void fpsimd_lazy_switch_to_guest(struct kvm_vcpu *vcpu)
{
u64 zcr_el1, zcr_el2;
if (!guest_owns_fp_regs())
return;
if (vcpu_has_sve(vcpu)) {
/* A guest hypervisor may restrict the effective max VL. */
if (vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu))
zcr_el2 = __vcpu_sys_reg(vcpu, ZCR_EL2);
else
zcr_el2 = vcpu_sve_max_vq(vcpu) - 1;
write_sysreg_el2(zcr_el2, SYS_ZCR);
zcr_el1 = __vcpu_sys_reg(vcpu, vcpu_sve_zcr_elx(vcpu));
write_sysreg_el1(zcr_el1, SYS_ZCR);
}
}
static inline void fpsimd_lazy_switch_to_host(struct kvm_vcpu *vcpu)
{
u64 zcr_el1, zcr_el2;
if (!guest_owns_fp_regs())
return;
/*
* When the guest owns the FP regs, we know that guest+hyp traps for
* any FPSIMD/SVE/SME features exposed to the guest have been disabled
* by either fpsimd_lazy_switch_to_guest() or kvm_hyp_handle_fpsimd()
* prior to __guest_entry(). As __guest_entry() guarantees a context
* synchronization event, we don't need an ISB here to avoid taking
* traps for anything that was exposed to the guest.
*/
if (vcpu_has_sve(vcpu)) {
zcr_el1 = read_sysreg_el1(SYS_ZCR);
__vcpu_sys_reg(vcpu, vcpu_sve_zcr_elx(vcpu)) = zcr_el1;
/*
* The guest's state is always saved using the guest's max VL.
* Ensure that the host has the guest's max VL active such that
* the host can save the guest's state lazily, but don't
* artificially restrict the host to the guest's max VL.
*/
if (has_vhe()) {
zcr_el2 = vcpu_sve_max_vq(vcpu) - 1;
write_sysreg_el2(zcr_el2, SYS_ZCR);
} else {
zcr_el2 = sve_vq_from_vl(kvm_host_sve_max_vl) - 1;
write_sysreg_el2(zcr_el2, SYS_ZCR);
zcr_el1 = vcpu_sve_max_vq(vcpu) - 1;
write_sysreg_el1(zcr_el1, SYS_ZCR);
}
}
}
static void kvm_hyp_save_fpsimd_host(struct kvm_vcpu *vcpu)
{
/*

13
arch/arm64/kvm/hyp/nvhe/hyp-main.c
View file

@ -5,6 +5,7 @@
*/
#include <hyp/adjust_pc.h>
#include <hyp/switch.h>
#include <asm/pgtable-types.h>
#include <asm/kvm_asm.h>
@ -224,8 +225,12 @@ static void handle___kvm_vcpu_run(struct kvm_cpu_context *host_ctxt)
sync_hyp_vcpu(hyp_vcpu);
} else {
struct kvm_vcpu *vcpu = kern_hyp_va(host_vcpu);
/* The host is fully trusted, run its vCPU directly. */
ret = __kvm_vcpu_run(kern_hyp_va(host_vcpu));
fpsimd_lazy_switch_to_guest(vcpu);
ret = __kvm_vcpu_run(vcpu);
fpsimd_lazy_switch_to_host(vcpu);
}
out:
cpu_reg(host_ctxt, 1) = ret;
@ -675,12 +680,6 @@ void handle_trap(struct kvm_cpu_context *host_ctxt)
case ESR_ELx_EC_SMC64:
handle_host_smc(host_ctxt);
break;
case ESR_ELx_EC_SVE:
cpacr_clear_set(0, CPACR_EL1_ZEN);
isb();
sve_cond_update_zcr_vq(sve_vq_from_vl(kvm_host_sve_max_vl) - 1,
SYS_ZCR_EL2);
break;
case ESR_ELx_EC_IABT_LOW:
case ESR_ELx_EC_DABT_LOW:
handle_host_mem_abort(host_ctxt);

6
arch/arm64/kvm/hyp/nvhe/switch.c
View file

@ -73,12 +73,10 @@ static void __activate_cptr_traps(struct kvm_vcpu *vcpu)
static void __deactivate_cptr_traps(struct kvm_vcpu *vcpu)
{
struct kvm *kvm = kern_hyp_va(vcpu->kvm);
if (has_hvhe()) {
u64 val = CPACR_EL1_FPEN;
if (!kvm_has_sve(kvm) || !guest_owns_fp_regs())
if (cpus_have_final_cap(ARM64_SVE))
val |= CPACR_EL1_ZEN;
if (cpus_have_final_cap(ARM64_SME))
val |= CPACR_EL1_SMEN;
@ -87,7 +85,7 @@ static void __deactivate_cptr_traps(struct kvm_vcpu *vcpu)
} else {
u64 val = CPTR_NVHE_EL2_RES1;
if (kvm_has_sve(kvm) && guest_owns_fp_regs())
if (!cpus_have_final_cap(ARM64_SVE))
val |= CPTR_EL2_TZ;
if (!cpus_have_final_cap(ARM64_SME))
val |= CPTR_EL2_TSM;

4
arch/arm64/kvm/hyp/vhe/switch.c
View file

@ -579,6 +579,8 @@ static int __kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
sysreg_save_host_state_vhe(host_ctxt);
fpsimd_lazy_switch_to_guest(vcpu);
/*
* Note that ARM erratum 1165522 requires us to configure both stage 1
* and stage 2 translation for the guest context before we clear
@ -603,6 +605,8 @@ static int __kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
__deactivate_traps(vcpu);
fpsimd_lazy_switch_to_host(vcpu);
sysreg_restore_host_state_vhe(host_ctxt);
if (guest_owns_fp_regs())