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KVM: PPC: Book3S HV P9: Reduce irq_work vs guest decrementer races 

irq_work's use of the DEC SPR is racy with guest<->host switch and guest
entry which flips the DEC interrupt to guest, which could lose a host
work interrupt.

This patch closes one race, and attempts to comment another class of
races.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210528090752.3542186-11-npiggin@gmail.com
This commit is contained in:
Nicholas Piggin 2021-05-28 19:07:30 +10:00 committed by Michael Ellerman
parent 413679e73b
commit 6ffe2c6e6d
3 changed files with 27 additions and 10 deletions
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12
arch/powerpc/include/asm/time.h
View File

@ -97,6 +97,18 @@ extern void div128_by_32(u64 dividend_high, u64 dividend_low,
extern void secondary_cpu_time_init(void);
extern void __init time_init(void);
#ifdef CONFIG_PPC64
static inline unsigned long test_irq_work_pending(void)
{
unsigned long x;
asm volatile("lbz %0,%1(13)"
: "=r" (x)
: "i" (offsetof(struct paca_struct, irq_work_pending)));
return x;
}
#endif
DECLARE_PER_CPU(u64, decrementers_next_tb);
/* Convert timebase ticks to nanoseconds */

10
arch/powerpc/kernel/time.c
View File

@ -508,16 +508,6 @@ EXPORT_SYMBOL(profile_pc);
* 64-bit uses a byte in the PACA, 32-bit uses a per-cpu variable...
*/
#ifdef CONFIG_PPC64
static inline unsigned long test_irq_work_pending(void)
{
unsigned long x;
asm volatile("lbz %0,%1(13)"
: "=r" (x)
: "i" (offsetof(struct paca_struct, irq_work_pending)));
return x;
}
static inline void set_irq_work_pending_flag(void)
{
asm volatile("stb %0,%1(13)" : :

15
arch/powerpc/kvm/book3s_hv.c
View File

@ -3708,6 +3708,18 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
if (!(vcpu->arch.ctrl & 1))
mtspr(SPRN_CTRLT, mfspr(SPRN_CTRLF) & ~1);
/*
* When setting DEC, we must always deal with irq_work_raise via NMI vs
* setting DEC. The problem occurs right as we switch into guest mode
* if a NMI hits and sets pending work and sets DEC, then that will
* apply to the guest and not bring us back to the host.
*
* irq_work_raise could check a flag (or possibly LPCR[HDICE] for
* example) and set HDEC to 1? That wouldn't solve the nested hv
* case which needs to abort the hcall or zero the time limit.
*
* XXX: Another day's problem.
*/
mtspr(SPRN_DEC, vcpu->arch.dec_expires - mftb());
if (kvmhv_on_pseries()) {
@ -3822,6 +3834,9 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
vc->in_guest = 0;
mtspr(SPRN_DEC, local_paca->kvm_hstate.dec_expires - mftb());
/* We may have raced with new irq work */
if (test_irq_work_pending())
set_dec(1);
mtspr(SPRN_SPRG_VDSO_WRITE, local_paca->sprg_vdso);
kvmhv_load_host_pmu();