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pvclock: Export a vDSO page even without rdtscp available

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When the cycle counter is "stable", i.e., synchronized across vCPUs by
the hypervisor, userspace can use a serialized rdtsc instead of relying
on rdtscp, just like the kernel timecounter does.  This can be useful
for performance in guests where the hypervisor hides rdtscp for some
reason.

To avoid breaking compatibility with older userspace which expects
rdtscp to be usable when pvclock exports timekeeping info, hide this
feature behind a sysctl.

Reviewed by:	kib
Tested by:	Shrikanth R Kamath <kshrikanth@juniper.net>
MFC after:	2 weeks
Sponsored by:	Klara, Inc.
Sponsored by:	Juniper Networks, Inc.
Differential Revision:	https://reviews.freebsd.org/D38342
This commit is contained in:
Mark Johnston 2023-02-03 10:54:23 -05:00
parent 26d105199e
commit 2bed14192c
2 changed files with 25 additions and 2 deletions
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1
sys/x86/include/pvclock.h
View File

@ -121,6 +121,7 @@ struct pvclock {
/* Private; initialized by the 'pvclock' API: */
bool vdso_force_unstable;
bool vdso_enable_without_rdtscp;
struct timecounter tc;
struct cdev *cdev;
};

26
sys/x86/x86/pvclock.c
View File

@ -224,6 +224,9 @@ pvclock_tc_vdso_timehands(struct vdso_timehands *vdso_th,
{
struct pvclock *pvc = tc->tc_priv;
if (pvc->cdev == NULL)
return (0);
vdso_th->th_algo = VDSO_TH_ALGO_X86_PVCLK;
vdso_th->th_x86_shift = 0;
vdso_th->th_x86_hpet_idx = 0;
@ -232,7 +235,9 @@ pvclock_tc_vdso_timehands(struct vdso_timehands *vdso_th,
vdso_th->th_x86_pvc_stable_mask = !pvc->vdso_force_unstable &&
pvc->stable_flag_supported ? PVCLOCK_FLAG_TSC_STABLE : 0;
bzero(vdso_th->th_res, sizeof(vdso_th->th_res));
return (pvc->cdev != NULL && amd_feature & AMDID_RDTSCP);
return ((amd_feature & AMDID_RDTSCP) != 0 ||
((vdso_th->th_x86_pvc_stable_mask & PVCLOCK_FLAG_TSC_STABLE) != 0 &&
pvc->vdso_enable_without_rdtscp));
}
#ifdef COMPAT_FREEBSD32
@ -242,6 +247,9 @@ pvclock_tc_vdso_timehands32(struct vdso_timehands32 *vdso_th,
{
struct pvclock *pvc = tc->tc_priv;
if (pvc->cdev == NULL)
return (0);
vdso_th->th_algo = VDSO_TH_ALGO_X86_PVCLK;
vdso_th->th_x86_shift = 0;
vdso_th->th_x86_hpet_idx = 0;
@ -250,7 +258,9 @@ pvclock_tc_vdso_timehands32(struct vdso_timehands32 *vdso_th,
vdso_th->th_x86_pvc_stable_mask = !pvc->vdso_force_unstable &&
pvc->stable_flag_supported ? PVCLOCK_FLAG_TSC_STABLE : 0;
bzero(vdso_th->th_res, sizeof(vdso_th->th_res));
return (pvc->cdev != NULL && amd_feature & AMDID_RDTSCP);
return ((amd_feature & AMDID_RDTSCP) != 0 ||
((vdso_th->th_x86_pvc_stable_mask & PVCLOCK_FLAG_TSC_STABLE) != 0 &&
pvc->vdso_enable_without_rdtscp));
}
#endif
@ -284,6 +294,18 @@ pvclock_init(struct pvclock *pvc, device_t dev, const char *tc_name,
"vdso_force_unstable", CTLFLAG_RW, &pvc->vdso_force_unstable, 0,
"Forcibly deassert stable flag in vDSO codepath");
/*
* Make it possible to use the vDSO page even when the hypervisor does
* not support the rdtscp instruction. This is disabled by default for
* compatibility with old libc.
*/
pvc->vdso_enable_without_rdtscp = false;
SYSCTL_ADD_BOOL(device_get_sysctl_ctx(dev),
SYSCTL_CHILDREN(device_get_sysctl_tree(dev)), OID_AUTO,
"vdso_enable_without_rdtscp", CTLFLAG_RWTUN,
&pvc->vdso_enable_without_rdtscp, 0,
"Allow the use of a vDSO when rdtscp is not available");
/* Set up timecounter and timecounter-supporting members: */
pvc->tc.tc_get_timecount = pvclock_tc_get_timecount;
pvc->tc.tc_poll_pps = NULL;