From 6d59d7a9f5b723a7ac1925c136e93ec83c0c3043 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Thu, 16 Feb 2012 13:33:12 -0800 Subject: i387: don't ever touch TS_USEDFPU directly, use helper functions This creates three helper functions that do the TS_USEDFPU accesses, and makes everybody that used to do it by hand use those helpers instead. In addition, there's a couple of helper functions for the "change both CR0.TS and TS_USEDFPU at the same time" case, and the places that do that together have been changed to use those. That means that we have fewer random places that open-code this situation. The intent is partly to clarify the code without actually changing any semantics yet (since we clearly still have some hard to reproduce bug in this area), but also to make it much easier to use another approach entirely to caching the CR0.TS bit for software accesses. Right now we use a bit in the thread-info 'status' variable (this patch does not change that), but we might want to make it a full field of its own or even make it a per-cpu variable. Signed-off-by: Linus Torvalds --- arch/x86/kvm/vmx.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'arch/x86/kvm/vmx.c') diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index d29216c..36091dd 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -1457,7 +1457,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx) #ifdef CONFIG_X86_64 wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base); #endif - if (current_thread_info()->status & TS_USEDFPU) + if (__thread_has_fpu(current_thread_info())) clts(); load_gdt(&__get_cpu_var(host_gdt)); } -- cgit v1.1 From f94edacf998516ac9d849f7bc6949a703977a7f3 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Fri, 17 Feb 2012 21:48:54 -0800 Subject: i387: move TS_USEDFPU flag from thread_info to task_struct This moves the bit that indicates whether a thread has ownership of the FPU from the TS_USEDFPU bit in thread_info->status to a word of its own (called 'has_fpu') in task_struct->thread.has_fpu. This fixes two independent bugs at the same time: - changing 'thread_info->status' from the scheduler causes nasty problems for the other users of that variable, since it is defined to be thread-synchronous (that's what the "TS_" part of the naming was supposed to indicate). So perfectly valid code could (and did) do ti->status |= TS_RESTORE_SIGMASK; and the compiler was free to do that as separate load, or and store instructions. Which can cause problems with preemption, since a task switch could happen in between, and change the TS_USEDFPU bit. The change to TS_USEDFPU would be overwritten by the final store. In practice, this seldom happened, though, because the 'status' field was seldom used more than once, so gcc would generally tend to generate code that used a read-modify-write instruction and thus happened to avoid this problem - RMW instructions are naturally low fat and preemption-safe. - On x86-32, the current_thread_info() pointer would, during interrupts and softirqs, point to a *copy* of the real thread_info, because x86-32 uses %esp to calculate the thread_info address, and thus the separate irq (and softirq) stacks would cause these kinds of odd thread_info copy aliases. This is normally not a problem, since interrupts aren't supposed to look at thread information anyway (what thread is running at interrupt time really isn't very well-defined), but it confused the heck out of irq_fpu_usable() and the code that tried to squirrel away the FPU state. (It also caused untold confusion for us poor kernel developers). It also turns out that using 'task_struct' is actually much more natural for most of the call sites that care about the FPU state, since they tend to work with the task struct for other reasons anyway (ie scheduling). And the FPU data that we are going to save/restore is found there too. Thanks to Arjan Van De Ven for pointing us to the %esp issue. Cc: Arjan van de Ven Reported-and-tested-by: Raphael Prevost Acked-and-tested-by: Suresh Siddha Tested-by: Peter Anvin Signed-off-by: Linus Torvalds --- arch/x86/kvm/vmx.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'arch/x86/kvm/vmx.c') diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index 36091dd..3b4c8d8 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -1457,7 +1457,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx) #ifdef CONFIG_X86_64 wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base); #endif - if (__thread_has_fpu(current_thread_info())) + if (__thread_has_fpu(current)) clts(); load_gdt(&__get_cpu_var(host_gdt)); } -- cgit v1.1