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SubjectRe: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature
On Fri, Jan 05, 2018 at 06:12:19PM -0800, Tim Chen wrote:
> +static ssize_t ibrs_enabled_write(struct file *file,
> + const char __user *user_buf,
> + size_t count, loff_t *ppos)
> +{
> + char buf[32];
> + ssize_t len;
> + unsigned int enable;
> +
> + len = min(count, sizeof(buf) - 1);
> + if (copy_from_user(buf, user_buf, len))
> + return -EFAULT;
> +
> + buf[len] = '\0';
> + if (kstrtouint(buf, 0, &enable))
> + return -EINVAL;
> +
> + if (enable > IBRS_MAX)
> + return -EINVAL;
> +
> + if (!boot_cpu_has(X86_FEATURE_SPEC_CTRL)) {
> + ibrs_enabled = IBRS_DISABLED;
> + return -EINVAL;
> + }
> +
> + mutex_lock(&spec_ctrl_mutex);
> +
> + if (enable == IBRS_DISABLED) {
> + /* disable IBRS usage */
> + ibrs_admin_disabled = true;
> + dynamic_ibrs = 0;
> + spec_ctrl_flush_all_cpus(MSR_IA32_SPEC_CTRL,
> + SPEC_CTRL_FEATURE_DISABLE_IBRS);
> +
> + } else if (enable == IBRS_ENABLED) {
> + /* enable IBRS usage in kernel */
> + ibrs_admin_disabled = false;
> + dynamic_ibrs = 1;

I think you need to do:

spec_ctrl_flush_all_cpus(MSR_IA32_SPEC_CTRL,
SPEC_CTRL_FEATURE_ENABLE_IBRS);

here as well, to force all CPUs into a known state.

> +
> + } else if (enable == IBRS_ENABLED_USER) {
> + /* enable IBRS all the time in both userspace and kernel */
> + ibrs_admin_disabled = false;
> + dynamic_ibrs = 0;
> + spec_ctrl_flush_all_cpus(MSR_IA32_SPEC_CTRL,
> + SPEC_CTRL_FEATURE_ENABLE_IBRS);
> + }
> +
> + ibrs_enabled = enable;
> +
> + mutex_unlock(&spec_ctrl_mutex);
> + return count;
> +}

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