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KVM: arm64: Preserve PSTATE.SS for the guest while single-step is enabled 

Preserve the PSTATE.SS value for the guest while userspace enables
single-step (i.e. while KVM manipulates the PSTATE.SS) for the vCPU.

Currently, while userspace enables single-step for the vCPU
(with KVM_GUESTDBG_SINGLESTEP), KVM sets PSTATE.SS to 1 on every
guest entry, not saving its original value.
When userspace disables single-step, KVM doesn't restore the original
value for the subsequent guest entry (use the current value instead).
Exception return instructions copy PSTATE.SS from SPSR_ELx.SS
only in certain cases when single-step is enabled (and set it to 0
in other cases). So, the value matters only when the guest enables
single-step (and when the guest's Software step state isn't affected
by single-step enabled by userspace, practically), though.

Fix this by preserving the original PSTATE.SS value while userspace
enables single-step, and restoring the value once it is disabled.

This fix modifies the behavior of GET_ONE_REG/SET_ONE_REG for the
PSTATE.SS while single-step is enabled by userspace.
Presently, GET_ONE_REG/SET_ONE_REG gets/sets the current PSTATE.SS
value, which KVM will override on the next guest entry (i.e. the
value userspace gets/sets is not used for the next guest entry).
With this patch, GET_ONE_REG/SET_ONE_REG will get/set the guest's
preserved value, which KVM will preserve and try to restore after
single-step is disabled.

Fixes: 337b99bf7e ("KVM: arm64: guest debug, add support for single-step")
Signed-off-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220917010600.532642-2-reijiw@google.com
This commit is contained in:
Reiji Watanabe 2022-09-16 18:05:57 -07:00 committed by Marc Zyngier
parent b90cb10531
commit 34fbdee086
2 changed files with 13 additions and 0 deletions
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1
arch/arm64/include/asm/kvm_host.h
View File

@ -393,6 +393,7 @@ struct kvm_vcpu_arch {
*/
struct {
u32 mdscr_el1;
bool pstate_ss;
} guest_debug_preserved;
/* vcpu power state */

12
arch/arm64/kvm/debug.c
View File

@ -32,6 +32,10 @@ static DEFINE_PER_CPU(u64, mdcr_el2);
*
* Guest access to MDSCR_EL1 is trapped by the hypervisor and handled
* after we have restored the preserved value to the main context.
*
* When single-step is enabled by userspace, we tweak PSTATE.SS on every
* guest entry. Preserve PSTATE.SS so we can restore the original value
* for the vcpu after the single-step is disabled.
*/
static void save_guest_debug_regs(struct kvm_vcpu *vcpu)
{
@ -41,6 +45,9 @@ static void save_guest_debug_regs(struct kvm_vcpu *vcpu)
trace_kvm_arm_set_dreg32("Saved MDSCR_EL1",
vcpu->arch.guest_debug_preserved.mdscr_el1);
vcpu->arch.guest_debug_preserved.pstate_ss =
(*vcpu_cpsr(vcpu) & DBG_SPSR_SS);
}
static void restore_guest_debug_regs(struct kvm_vcpu *vcpu)
@ -51,6 +58,11 @@ static void restore_guest_debug_regs(struct kvm_vcpu *vcpu)
trace_kvm_arm_set_dreg32("Restored MDSCR_EL1",
vcpu_read_sys_reg(vcpu, MDSCR_EL1));
if (vcpu->arch.guest_debug_preserved.pstate_ss)
*vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
else
*vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
}
/**