| Message ID | 1411384668-11135-1-git-send-email-pbonzini@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon, Sep 22, 2014 at 01:17:48PM +0200, Paolo Bonzini wrote:
> On x86_64, kernel text mappings are mapped read-only with CONFIG_DEBUG_RODATA.
Hmm, that depends on DEBUG_KERNEL.
I think you're actually talking about distro kernels which enable
CONFIG_DEBUG_RODATA, right?
Il 22/09/2014 21:43, Borislav Petkov ha scritto: >> > On x86_64, kernel text mappings are mapped read-only with CONFIG_DEBUG_RODATA. > Hmm, that depends on DEBUG_KERNEL. > > I think you're actually talking about distro kernels which enable > CONFIG_DEBUG_RODATA, right? This is for guest kernels, so it's not necessarily distro kernels. Anyone who compiles their kernel with CONFIG_DEBUG_RODATA + PV spinlocks would not be able to run it on AMD. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Tue, Sep 23, 2014 at 10:00:12AM +0200, Paolo Bonzini wrote: > Il 22/09/2014 21:43, Borislav Petkov ha scritto: > >> > On x86_64, kernel text mappings are mapped read-only with CONFIG_DEBUG_RODATA. > > Hmm, that depends on DEBUG_KERNEL. > > > > I think you're actually talking about distro kernels which enable > > CONFIG_DEBUG_RODATA, right? > > This is for guest kernels, so it's not necessarily distro kernels. > Anyone who compiles their kernel with CONFIG_DEBUG_RODATA + PV spinlocks > would not be able to run it on AMD. I see. Yeah, so the patch makes sense to me: Acked-by: Borislav Petkov <bp@suse.de> Thanks.
On Mon, 22 Sep 2014, Paolo Bonzini wrote: > On x86_64, kernel text mappings are mapped read-only with CONFIG_DEBUG_RODATA. > In that case, KVM will fail to patch VMCALL instructions to VMMCALL > as required on AMD processors. > > The failure mode is currently a divide-by-zero exception, which obviously > is a KVM bug that has to be fixed. However, picking the right instruction > between VMCALL and VMMCALL will be faster and will help if you cannot upgrade > the hypervisor. > > -/* This instruction is vmcall. On non-VT architectures, it will generate a > - * trap that we will then rewrite to the appropriate instruction. > +#ifdef CONFIG_DEBUG_RODATA > +#define KVM_HYPERCALL \ > + ALTERNATIVE(".byte 0x0f,0x01,0xc1", ".byte 0x0f,0x01,0xd9", X86_FEATURE_VMMCALL) If we can do it via a feature bit and alternatives, then why do you want to patch it manually if CONFIG_DEBUG_RODATA=n? Just because more #ifdeffery makes the code more readable? Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h index bb9b258d60e7..2075e6c34c78 100644 --- a/arch/x86/include/asm/cpufeature.h +++ b/arch/x86/include/asm/cpufeature.h @@ -202,6 +202,7 @@ #define X86_FEATURE_DECODEASSISTS ( 8*32+12) /* AMD Decode Assists support */ #define X86_FEATURE_PAUSEFILTER ( 8*32+13) /* AMD filtered pause intercept */ #define X86_FEATURE_PFTHRESHOLD ( 8*32+14) /* AMD pause filter threshold */ +#define X86_FEATURE_VMMCALL ( 8*32+15) /* Prefer vmmcall to vmcall */ /* Intel-defined CPU features, CPUID level 0x00000007:0 (ebx), word 9 */ diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h index c7678e43465b..e62cf897f781 100644 --- a/arch/x86/include/asm/kvm_para.h +++ b/arch/x86/include/asm/kvm_para.h @@ -2,6 +2,7 @@ #define _ASM_X86_KVM_PARA_H #include <asm/processor.h> +#include <asm/alternative.h> #include <uapi/asm/kvm_para.h> extern void kvmclock_init(void); @@ -16,10 +17,15 @@ static inline bool kvm_check_and_clear_guest_paused(void) } #endif /* CONFIG_KVM_GUEST */ -/* This instruction is vmcall. On non-VT architectures, it will generate a - * trap that we will then rewrite to the appropriate instruction. +#ifdef CONFIG_DEBUG_RODATA +#define KVM_HYPERCALL \ + ALTERNATIVE(".byte 0x0f,0x01,0xc1", ".byte 0x0f,0x01,0xd9", X86_FEATURE_VMMCALL) +#else +/* On AMD processors, vmcall will generate a trap that we will + * then rewrite to the appropriate instruction. */ #define KVM_HYPERCALL ".byte 0x0f,0x01,0xc1" +#endif /* For KVM hypercalls, a three-byte sequence of either the vmcall or the vmmcall * instruction. The hypervisor may replace it with something else but only the diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c index 60e5497681f5..813d29d00a17 100644 --- a/arch/x86/kernel/cpu/amd.c +++ b/arch/x86/kernel/cpu/amd.c @@ -525,6 +525,13 @@ static void early_init_amd(struct cpuinfo_x86 *c) } #endif + /* + * This is only needed to tell the kernel whether to use VMCALL + * and VMMCALL. VMMCALL is never executed except under virt, so + * we can set it unconditionally. + */ + set_cpu_cap(c, X86_FEATURE_VMMCALL); + /* F16h erratum 793, CVE-2013-6885 */ if (c->x86 == 0x16 && c->x86_model <= 0xf) msr_set_bit(MSR_AMD64_LS_CFG, 15);
On x86_64, kernel text mappings are mapped read-only with CONFIG_DEBUG_RODATA. In that case, KVM will fail to patch VMCALL instructions to VMMCALL as required on AMD processors. The failure mode is currently a divide-by-zero exception, which obviously is a KVM bug that has to be fixed. However, picking the right instruction between VMCALL and VMMCALL will be faster and will help if you cannot upgrade the hypervisor. Reported-by: Chris Webb <chris@arachsys.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@redhat.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: x86@kernel.org Cc: Borislav Petkov <bp@suse.de> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> --- arch/x86/include/asm/cpufeature.h | 1 + arch/x86/include/asm/kvm_para.h | 10 ++++++++-- arch/x86/kernel/cpu/amd.c | 7 +++++++ 3 files changed, 16 insertions(+), 2 deletions(-)