Messages in this thread | | | Date | Mon, 17 Oct 2022 15:39:39 -0700 | Subject | Re: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for init_fpstate | From | "Chang S. Bae" <> |
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On 10/11/2022 3:24 PM, Dave Hansen wrote: > > diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c > index 3b28c5b25e12..4d64de34da12 100644 > --- a/arch/x86/kernel/fpu/core.c > +++ b/arch/x86/kernel/fpu/core.c > @@ -526,6 +526,9 @@ static void __fpstate_reset(struct fpstate *fpstate, u64 xfd) > fpstate->xfeatures = fpu_kernel_cfg.default_features; > fpstate->user_xfeatures = fpu_user_cfg.default_features; > fpstate->xfd = xfd; > + > + /* Ensure that xcomp_bv matches ->xfeatures */ > + xstate_init_xcomp_bv(&fpstate->regs.xsave, fpstate->xfeatures); > }
I have some difficulty understanding the problem without this. Maybe I'm missing something here:
We have two call sites for this -- (a) one for the guest fpstate allocation [1] and (b) the other for the reset [2].
(a) The former has a call chain to init xcomp_bv: fpu_alloc_guest_fpstate()->fpstate_init_user()->xstate_init_xcomp_bv()
(b) And the latter picks up the default area: void fpstate_reset(struct fpu *fpu) { /* Set the fpstate pointer to the default fpstate */ fpu->fpstate = &fpu->__fpstate; __fpstate_reset(fpu->fpstate, init_fpstate.xfd); ... } Then, xcomp_bv looks to be subsequently written by XSAVE* or by copying from init_fpstate.
There are three distinct call sites for fpstate_reset(): * fpu_clone() [3]: the child will either copy from init_fpstate or do XSAVE* that will update xcomp_bv according to ->xfeatures. * fpu__init_system_xstate() [4]: When the init task switches away, xcomp_bv will be updated by XSAVE*. * fpu_flush_thread() [5]: xcomp_bv will be copied from init_fpstate via fpu_reset_fpregs().
Thanks, Chang
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/core.c#n229 [2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/core.c#n535 [3]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/core.c#n567 [4]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/xstate.c#n869 [5]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/core.c#n744
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