Messages in this thread | | | From | Brian Gerst <> | Date | Wed, 2 Sep 2020 13:02:07 -0400 | Subject | Re: [PATCH 01/13] x86/entry: Fix AC assertion |
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On Wed, Sep 2, 2020 at 12:31 PM <peterz@infradead.org> wrote: > > On Wed, Sep 02, 2020 at 06:24:27PM +0200, Jürgen Groß wrote: > > On 02.09.20 17:58, Brian Gerst wrote: > > > On Wed, Sep 2, 2020 at 9:38 AM Peter Zijlstra <peterz@infradead.org> wrote: > > > > > > > > From: Peter Zijlstra <peterz@infradead.org> > > > > > > > > The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further > > > > improve user entry sanity checks") unconditionally triggers on my IVB > > > > machine because it does not support SMAP. > > > > > > > > For !SMAP hardware we patch out CLAC/STAC instructions and thus if > > > > userspace sets AC, we'll still have it set after entry. > > > > > > > > Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks") > > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > > > > Acked-by: Andy Lutomirski <luto@kernel.org> > > > > --- > > > > arch/x86/include/asm/entry-common.h | 11 +++++++++-- > > > > 1 file changed, 9 insertions(+), 2 deletions(-) > > > > > > > > --- a/arch/x86/include/asm/entry-common.h > > > > +++ b/arch/x86/include/asm/entry-common.h > > > > @@ -18,8 +18,16 @@ static __always_inline void arch_check_u > > > > * state, not the interrupt state as imagined by Xen. > > > > */ > > > > unsigned long flags = native_save_fl(); > > > > - WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF | > > > > - X86_EFLAGS_NT)); > > > > + unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT; > > > > + > > > > + /* > > > > + * For !SMAP hardware we patch out CLAC on entry. > > > > + */ > > > > + if (boot_cpu_has(X86_FEATURE_SMAP) || > > > > + (IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV))) > > > > + mask |= X86_EFLAGS_AC; > > > > > > Is the explicit Xen check necessary? IIRC the Xen hypervisor will > > > filter out the SMAP bit in the cpuid pvop. > > > > Right, and this test will nevertheless result in setting AC in the mask. > > IIRC this was the main objective here. > > Correct, this asserts that 64bit Xen-PV will never have AC set; it had > better not have it set since it runs in ring 3.
Ok. That should be added to the comment to avoid confusion.
-- Brian Gerst
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