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    Subject[PATCH 5.10 012/152] drm/i915: Allow the sysadmin to override security mitigations
    Date
    From: Chris Wilson <chris@chris-wilson.co.uk>

    commit 984cadea032b103c5824a5f29d0a36b3e9df6333 upstream.

    The clear-residuals mitigation is a relatively heavy hammer and under some
    circumstances the user may wish to forgo the context isolation in order
    to meet some performance requirement. Introduce a generic module
    parameter to allow selectively enabling/disabling different mitigations.

    To disable just the clear-residuals mitigation (on Ivybridge, Baytrail,
    or Haswell) use the module parameter: i915.mitigations=auto,!residuals

    Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1858
    Fixes: 47f8253d2b89 ("drm/i915/gen7: Clear all EU/L3 residual contexts")
    Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
    Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
    Cc: Jon Bloomfield <jon.bloomfield@intel.com>
    Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
    Cc: stable@vger.kernel.org # v5.7
    Reviewed-by: Jon Bloomfield <jon.bloomfield@intel.com>
    Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20210111225220.3483-3-chris@chris-wilson.co.uk
    (cherry picked from commit f7452c7cbd5b5dfb9a6c84cb20bea04c89be50cd)
    Signed-off-by: Jani Nikula <jani.nikula@intel.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/gpu/drm/i915/Makefile | 1
    drivers/gpu/drm/i915/gt/intel_ring_submission.c | 4
    drivers/gpu/drm/i915/i915_mitigations.c | 146 ++++++++++++++++++++++++
    drivers/gpu/drm/i915/i915_mitigations.h | 13 ++
    4 files changed, 163 insertions(+), 1 deletion(-)

    --- a/drivers/gpu/drm/i915/Makefile
    +++ b/drivers/gpu/drm/i915/Makefile
    @@ -38,6 +38,7 @@ i915-y += i915_drv.o \
    i915_config.o \
    i915_irq.o \
    i915_getparam.o \
    + i915_mitigations.o \
    i915_params.o \
    i915_pci.o \
    i915_scatterlist.o \
    --- a/drivers/gpu/drm/i915/gt/intel_ring_submission.c
    +++ b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
    @@ -32,6 +32,7 @@
    #include "gen6_ppgtt.h"
    #include "gen7_renderclear.h"
    #include "i915_drv.h"
    +#include "i915_mitigations.h"
    #include "intel_breadcrumbs.h"
    #include "intel_context.h"
    #include "intel_gt.h"
    @@ -885,7 +886,8 @@ static int switch_context(struct i915_re
    GEM_BUG_ON(HAS_EXECLISTS(engine->i915));

    if (engine->wa_ctx.vma && ce != engine->kernel_context) {
    - if (engine->wa_ctx.vma->private != ce) {
    + if (engine->wa_ctx.vma->private != ce &&
    + i915_mitigate_clear_residuals()) {
    ret = clear_residuals(rq);
    if (ret)
    return ret;
    --- /dev/null
    +++ b/drivers/gpu/drm/i915/i915_mitigations.c
    @@ -0,0 +1,146 @@
    +// SPDX-License-Identifier: MIT
    +/*
    + * Copyright © 2021 Intel Corporation
    + */
    +
    +#include <linux/kernel.h>
    +#include <linux/moduleparam.h>
    +#include <linux/slab.h>
    +#include <linux/string.h>
    +
    +#include "i915_drv.h"
    +#include "i915_mitigations.h"
    +
    +static unsigned long mitigations __read_mostly = ~0UL;
    +
    +enum {
    + CLEAR_RESIDUALS = 0,
    +};
    +
    +static const char * const names[] = {
    + [CLEAR_RESIDUALS] = "residuals",
    +};
    +
    +bool i915_mitigate_clear_residuals(void)
    +{
    + return READ_ONCE(mitigations) & BIT(CLEAR_RESIDUALS);
    +}
    +
    +static int mitigations_set(const char *val, const struct kernel_param *kp)
    +{
    + unsigned long new = ~0UL;
    + char *str, *sep, *tok;
    + bool first = true;
    + int err = 0;
    +
    + BUILD_BUG_ON(ARRAY_SIZE(names) >= BITS_PER_TYPE(mitigations));
    +
    + str = kstrdup(val, GFP_KERNEL);
    + if (!str)
    + return -ENOMEM;
    +
    + for (sep = str; (tok = strsep(&sep, ","));) {
    + bool enable = true;
    + int i;
    +
    + /* Be tolerant of leading/trailing whitespace */
    + tok = strim(tok);
    +
    + if (first) {
    + first = false;
    +
    + if (!strcmp(tok, "auto"))
    + continue;
    +
    + new = 0;
    + if (!strcmp(tok, "off"))
    + continue;
    + }
    +
    + if (*tok == '!') {
    + enable = !enable;
    + tok++;
    + }
    +
    + if (!strncmp(tok, "no", 2)) {
    + enable = !enable;
    + tok += 2;
    + }
    +
    + if (*tok == '\0')
    + continue;
    +
    + for (i = 0; i < ARRAY_SIZE(names); i++) {
    + if (!strcmp(tok, names[i])) {
    + if (enable)
    + new |= BIT(i);
    + else
    + new &= ~BIT(i);
    + break;
    + }
    + }
    + if (i == ARRAY_SIZE(names)) {
    + pr_err("Bad \"%s.mitigations=%s\", '%s' is unknown\n",
    + DRIVER_NAME, val, tok);
    + err = -EINVAL;
    + break;
    + }
    + }
    + kfree(str);
    + if (err)
    + return err;
    +
    + WRITE_ONCE(mitigations, new);
    + return 0;
    +}
    +
    +static int mitigations_get(char *buffer, const struct kernel_param *kp)
    +{
    + unsigned long local = READ_ONCE(mitigations);
    + int count, i;
    + bool enable;
    +
    + if (!local)
    + return scnprintf(buffer, PAGE_SIZE, "%s\n", "off");
    +
    + if (local & BIT(BITS_PER_LONG - 1)) {
    + count = scnprintf(buffer, PAGE_SIZE, "%s,", "auto");
    + enable = false;
    + } else {
    + enable = true;
    + count = 0;
    + }
    +
    + for (i = 0; i < ARRAY_SIZE(names); i++) {
    + if ((local & BIT(i)) != enable)
    + continue;
    +
    + count += scnprintf(buffer + count, PAGE_SIZE - count,
    + "%s%s,", enable ? "" : "!", names[i]);
    + }
    +
    + buffer[count - 1] = '\n';
    + return count;
    +}
    +
    +static const struct kernel_param_ops ops = {
    + .set = mitigations_set,
    + .get = mitigations_get,
    +};
    +
    +module_param_cb_unsafe(mitigations, &ops, NULL, 0600);
    +MODULE_PARM_DESC(mitigations,
    +"Selectively enable security mitigations for all Intel® GPUs in the system.\n"
    +"\n"
    +" auto -- enables all mitigations required for the platform [default]\n"
    +" off -- disables all mitigations\n"
    +"\n"
    +"Individual mitigations can be enabled by passing a comma-separated string,\n"
    +"e.g. mitigations=residuals to enable only clearing residuals or\n"
    +"mitigations=auto,noresiduals to disable only the clear residual mitigation.\n"
    +"Either '!' or 'no' may be used to switch from enabling the mitigation to\n"
    +"disabling it.\n"
    +"\n"
    +"Active mitigations for Ivybridge, Baytrail, Haswell:\n"
    +" residuals -- clear all thread-local registers between contexts"
    +);
    --- /dev/null
    +++ b/drivers/gpu/drm/i915/i915_mitigations.h
    @@ -0,0 +1,13 @@
    +/* SPDX-License-Identifier: MIT */
    +/*
    + * Copyright © 2021 Intel Corporation
    + */
    +
    +#ifndef __I915_MITIGATIONS_H__
    +#define __I915_MITIGATIONS_H__
    +
    +#include <linux/types.h>
    +
    +bool i915_mitigate_clear_residuals(void);
    +
    +#endif /* __I915_MITIGATIONS_H__ */

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