Re: [RFC PATCH 07/22] x86/fpu/xstate: Introduce helpers to manage an xstate area dynamically

[Andy Lutomirski]

On Thu, Oct 1, 2020 at 1:42 PM Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
>
> task->fpu has a buffer to keep the extended register states, but it is not
> expandable at runtime. Introduce runtime methods and new fpu struct fields
> to support the expansion.
>
> fpu->state_mask indicates the saved states per task and fpu->state_ptr
> points the dynamically allocated area.
>
> alloc_xstate_area() uses vmalloc() for its scalability. However, set a
> threshold (64KB) to watch out a potential need for an alternative
> mechanism.
>
> Also, introduce a new helper -- get_xstate_size() to calculate the area
> size.
>
> No functional change until the kernel supports dynamic user states.
>
> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  arch/x86/include/asm/fpu/types.h  |  29 +++++--
>  arch/x86/include/asm/fpu/xstate.h |   3 +
>  arch/x86/kernel/fpu/core.c        |   3 +
>  arch/x86/kernel/fpu/xstate.c      | 124 ++++++++++++++++++++++++++++++
>  4 files changed, 154 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
> index c87364ea6446..4b7756644824 100644
> --- a/arch/x86/include/asm/fpu/types.h
> +++ b/arch/x86/include/asm/fpu/types.h
> @@ -327,14 +327,33 @@ struct fpu {
>          */
>         unsigned long                   avx512_timestamp;
>
> +       /*
> +        * @state_mask:
> +        *
> +        * The state component bitmap. It indicates the saved xstate in
> +        * either @state or @state_ptr. The map value starts to be aligned
> +        * with @state and then with @state_ptr once it is in use.
> +        */
> +       u64                             state_mask;
> +
> +       /*
> +        * @state_ptr:
> +        *
> +        * Copy of all extended register states, in a dynamically-allocated
> +        * area, we save and restore over context switches. When a task is
> +        * using extended features, the register state is always the most
> +        * current. This state copy is more recent than @state. If the task
> +        * context-switches away, they get saved here, representing the xstate.
> +        */
> +       union fpregs_state              *state_ptr;
> +
>         /*
>          * @state:
>          *
> -        * In-memory copy of all FPU registers that we save/restore
> -        * over context switches. If the task is using the FPU then
> -        * the registers in the FPU are more recent than this state
> -        * copy. If the task context-switches away then they get
> -        * saved here and represent the FPU state.
> +        * Copy of some extended register state that we save and restore
> +        * over context switches. If a task uses a dynamically-allocated
> +        * area, @state_ptr, then it has a more recent state copy than this.
> +        * This copy follows the same attributes as described for @state_ptr.
>          */
>         union fpregs_state              state;
>         /*
> diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
> index 9aad91c0725b..37728bfcb71e 100644
> --- a/arch/x86/include/asm/fpu/xstate.h
> +++ b/arch/x86/include/asm/fpu/xstate.h
> @@ -103,6 +103,9 @@ extern void __init update_regset_xstate_info(unsigned int size,
>                                              u64 xstate_mask);
>
>  void *get_xsave_addr(struct fpu *fpu, int xfeature_nr);
> +int alloc_xstate_area(struct fpu *fpu, u64 mask, unsigned int *alloc_size);
> +void free_xstate_area(struct fpu *fpu);
> +
>  const void *get_xsave_field_ptr(int xfeature_nr);
>  int using_compacted_format(void);
>  int xfeature_size(int xfeature_nr);
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 875620fdfe61..e25f7866800e 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -235,6 +235,9 @@ int fpu__copy(struct task_struct *dst, struct task_struct *src)
>          */
>         memset(&dst_fpu->state.xsave, 0, fpu_kernel_xstate_default_size);
>
> +       dst_fpu->state_mask = xfeatures_mask_all & ~xfeatures_mask_user_dynamic;
> +       dst_fpu->state_ptr = NULL;
> +
>         /*
>          * If the FPU registers are not current just memcpy() the state.
>          * Otherwise save current FPU registers directly into the child's FPU
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 6e0d8a9699ed..af60332aafef 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -10,6 +10,7 @@
>  #include <linux/pkeys.h>
>  #include <linux/seq_file.h>
>  #include <linux/proc_fs.h>
> +#include <linux/vmalloc.h>
>
>  #include <asm/fpu/api.h>
>  #include <asm/fpu/internal.h>
> @@ -69,6 +70,7 @@ static unsigned int xstate_offsets[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] =
>  static unsigned int xstate_sizes[XFEATURE_MAX]   = { [ 0 ... XFEATURE_MAX - 1] = -1};
>  static unsigned int xstate_comp_offsets[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = -1};
>  static unsigned int xstate_supervisor_only_offsets[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = -1};
> +static bool xstate_aligns[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = false};
>
>  /*
>   * The XSAVE area of kernel can be in standard or compacted format;
> @@ -128,6 +130,48 @@ static bool xfeature_is_supervisor(int xfeature_nr)
>         return ecx & 1;
>  }
>
> +/*
> + * Available once those arrays for the offset, size, and alignment info are set up,
> + * by setup_xstate_features().
> + */
> +static unsigned int get_xstate_size(u64 mask)
> +{
> +       unsigned int size;
> +       u64 xmask;
> +       int i, nr;
> +
> +       if (!mask)
> +               return 0;
> +       else if (mask == (xfeatures_mask_all & ~xfeatures_mask_user_dynamic))
> +               return fpu_kernel_xstate_default_size;
> +       else if (mask == xfeatures_mask_all)
> +               return fpu_kernel_xstate_max_size;
> +
> +       nr = fls64(mask) - 1;
> +
> +       if (!using_compacted_format())
> +               return xstate_offsets[nr] + xstate_sizes[nr];
> +
> +       xmask = BIT_ULL(nr + 1) - 1;
> +
> +       if (mask == (xmask & xfeatures_mask_all))
> +               return xstate_comp_offsets[nr] + xstate_sizes[nr];
> +
> +       /*
> +        * Calculate the size by summing up each state together, since no known
> +        * size found with the xstate area format out of the given mask.
> +        */
> +       for (size = FXSAVE_SIZE + XSAVE_HDR_SIZE, i = FIRST_EXTENDED_XFEATURE; i <= nr; i++) {
> +               if (!(mask & BIT_ULL(i)))
> +                       continue;
> +
> +               if (xstate_aligns[i])
> +                       size = ALIGN(size, 64);
> +               size += xstate_sizes[i];
> +       }
> +       return size;
> +}
> +
>  /*
>   * When executing XSAVEOPT (or other optimized XSAVE instructions), if
>   * a processor implementation detects that an FPU state component is still
> @@ -268,10 +312,12 @@ static void __init setup_xstate_features(void)
>         xstate_offsets[XFEATURE_FP]     = 0;
>         xstate_sizes[XFEATURE_FP]       = offsetof(struct fxregs_state,
>                                                    xmm_space);
> +       xstate_aligns[XFEATURE_FP]      = true;
>
>         xstate_offsets[XFEATURE_SSE]    = xstate_sizes[XFEATURE_FP];
>         xstate_sizes[XFEATURE_SSE]      = sizeof_field(struct fxregs_state,
>                                                        xmm_space);
> +       xstate_aligns[XFEATURE_SSE]     = true;
>
>         for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
>                 if (!xfeature_enabled(i))
> @@ -289,6 +335,7 @@ static void __init setup_xstate_features(void)
>                         continue;
>
>                 xstate_offsets[i] = ebx;
> +               xstate_aligns[i] = (ecx & 2) ? true : false;
>
>                 /*
>                  * In our xstate size checks, we assume that the highest-numbered
> @@ -753,6 +800,9 @@ static bool is_supported_xstate_size(unsigned int test_xstate_size)
>         return false;
>  }
>
> +/* The watched threshold size of dynamically allocated xstate area */
> +#define XSTATE_AREA_MAX_BYTES          (64 * 1024)
> +
>  static int __init init_xstate_size(void)
>  {
>         /* Recompute the context size for enabled features: */
> @@ -777,6 +827,14 @@ static int __init init_xstate_size(void)
>         if (!is_supported_xstate_size(fpu_kernel_xstate_default_size))
>                 return -EINVAL;
>
> +       /*
> +        * When observing the allocation goes beyond the threshold, it is better to consider
> +        * switching a better scalable mechanism than the current.
> +        */
> +       if (fpu_kernel_xstate_max_size > XSTATE_AREA_MAX_BYTES)
> +               pr_warn("x86/fpu: xstate buffer too large (%u > %u)\n",
> +                       fpu_kernel_xstate_max_size, XSTATE_AREA_MAX_BYTES);
> +
>         /*
>          * User space is always in standard format.
>          */
> @@ -867,6 +925,12 @@ void __init fpu__init_system_xstate(void)
>         if (err)
>                 goto out_disable;
>
> +       /*
> +        * At this point, it passed the size sanity check. Have the init_task take
> +        * the feature mask.
> +        */
> +       current->thread.fpu.state_mask = (xfeatures_mask_all & ~xfeatures_mask_user_dynamic);
> +
>         /*
>          * Update info used for ptrace frames; use standard-format size and no
>          * supervisor xstates:
> @@ -1086,6 +1150,66 @@ static inline bool xfeatures_mxcsr_quirk(u64 xfeatures)
>         return true;
>  }
>
> +void free_xstate_area(struct fpu *fpu)
> +{
> +       vfree(fpu->state_ptr);
> +}
> +
> +/*
> + * Allocate a new xstate area with a calculated size, based on the given bit value.
> + *
> + * No mechanism implemented yet to shrink or reclaim the xstate area on the fly,
> + * the need of which is subject to the real usage.
> + */
> +int alloc_xstate_area(struct fpu *fpu, u64 mask, unsigned int *alloc_size)
> +{
> +       union fpregs_state *state_ptr;
> +       unsigned int oldsz, newsz;
> +       u64 state_mask;
> +
> +       state_mask = fpu->state_mask | mask;
> +
> +       oldsz = get_xstate_size(fpu->state_mask);
> +       newsz = get_xstate_size(state_mask);
> +
> +       if (oldsz >= newsz)
> +               return 0;
> +
> +       if (newsz > fpu_kernel_xstate_max_size) {
> +               pr_warn_once("x86/fpu: state buffer too large (%u > %u bytes)\n",
> +                            newsz, fpu_kernel_xstate_max_size);
> +               XSTATE_WARN_ON(1);
> +               return 0;
> +       }
> +
> +       /*
> +        * The caller may be under interrupt disabled condition. Ensure interrupt
> +        * allowance before the memory allocation, which may involve with page faults.
> +        */
> +       local_irq_enable();
> +       /* We need 64B aligned pointer, but vmalloc() returns a page-aligned address */
> +       state_ptr = vmalloc(newsz);

I'm speechless.

First, you can't just enable IRQs here.  If IRQs are off, they're off
for a reason.  Secondly, if they're *on*, you just forgot that fact.

And allocating this state from vmalloc() seems questionable.  Why are
you doing this?