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    SubjectRe: [PATCH v7 38/41] x86/fpu: Add helper for initing features
    On Mon, Feb 27, 2023 at 02:29:54PM -0800, Rick Edgecombe wrote:
    > Subject: Re: [PATCH v7 38/41] x86/fpu: Add helper for initing features

    "initializing"

    > If an xfeature is saved in a buffer, the xfeature's bit will be set in
    > xsave->header.xfeatures. The CPU may opt to not save the xfeature if it
    > is in it's init state. In this case the xfeature buffer address cannot

    "its"

    > be retrieved with get_xsave_addr().
    >
    > Future patches will need to handle the case of writing to an xfeature
    > that may not be saved. So provide helpers to init an xfeature in an
    > xsave buffer.
    >
    > This could of course be done directly by reaching into the xsave buffer,
    > however this would not be robust against future changes to optimize the
    > xsave buffer by compacting it. In that case the xsave buffer would need
    > to be re-arranged as well. So the logic properly belongs encapsulated
    > in a helper where the logic can be unified.
    >
    > Tested-by: Pengfei Xu <pengfei.xu@intel.com>
    > Tested-by: John Allen <john.allen@amd.com>
    > Tested-by: Kees Cook <keescook@chromium.org>
    > Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
    > Reviewed-by: Kees Cook <keescook@chromium.org>
    > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    >
    > ---
    > v2:
    > - New patch
    > ---
    > arch/x86/kernel/fpu/xstate.c | 58 +++++++++++++++++++++++++++++-------
    > arch/x86/kernel/fpu/xstate.h | 6 ++++
    > 2 files changed, 53 insertions(+), 11 deletions(-)
    >
    > diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
    > index 13a80521dd51..3ff80be0a441 100644
    > --- a/arch/x86/kernel/fpu/xstate.c
    > +++ b/arch/x86/kernel/fpu/xstate.c
    > @@ -934,6 +934,24 @@ static void *__raw_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
    > return (void *)xsave + xfeature_get_offset(xcomp_bv, xfeature_nr);
    > }
    >
    > +static int xsave_buffer_access_checks(int xfeature_nr)

    Function name needs a verb.

    > +{
    > + /*
    > + * Do we even *have* xsave state?
    > + */

    That comment is superfluous.

    > + if (!boot_cpu_has(X86_FEATURE_XSAVE))

    check_for_deprecated_apis: WARNING: arch/x86/kernel/fpu/xstate.c:942: Do not use boot_cpu_has() - use cpu_feature_enabled() instead.

    > + return 1;
    > +
    > + /*
    > + * We should not ever be requesting features that we

    Please use passive voice in your commit message: no "we" or "I", etc,
    and describe your changes in imperative mood.

    > + * have not enabled.
    > + */
    > + if (WARN_ON_ONCE(!xfeature_enabled(xfeature_nr)))
    > + return 1;
    > +
    > + return 0;
    > +}
    > +
    > /*
    > * Given the xsave area and a state inside, this function returns the
    > * address of the state.
    > @@ -954,17 +972,7 @@ static void *__raw_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
    > */
    > void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
    > {
    > - /*
    > - * Do we even *have* xsave state?
    > - */
    > - if (!boot_cpu_has(X86_FEATURE_XSAVE))
    > - return NULL;
    > -
    > - /*
    > - * We should not ever be requesting features that we
    > - * have not enabled.
    > - */
    > - if (WARN_ON_ONCE(!xfeature_enabled(xfeature_nr)))
    > + if (xsave_buffer_access_checks(xfeature_nr))
    > return NULL;
    >
    > /*
    > @@ -984,6 +992,34 @@ void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
    > return __raw_xsave_addr(xsave, xfeature_nr);
    > }
    >
    > +/*
    > + * Given the xsave area and a state inside, this function
    > + * initializes an xfeature in the buffer.

    s/this function initializes/initialize/

    > + *
    > + * get_xsave_addr() will return NULL if the feature bit is
    > + * not present in the header. This function will make it so
    > + * the xfeature buffer address is ready to be retrieved by
    > + * get_xsave_addr().

    So users of get_xsave_addr() would have to know that they would need to
    call init_xfeature()?

    I think the better approach would be:

    void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr, bool init)

    and then that @init controls whether get_xsave_addr() should init the
    buffer.

    And then you don't have to have a bunch of small functions here and
    there and know when to call what but get_xsave_addr() would simply DTRT.

    Thx.

    --
    Regards/Gruss,
    Boris.

    https://people.kernel.org/tglx/notes-about-netiquette

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