Commit 9a8d305b authored by Paolo Bonzini's avatar Paolo Bonzini Committed by Ben Hutchings

x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is read-only

commit c1118b36 upstream.

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: default avatarChris Webb <chris@arachsys.com>
Tested-by: default avatarChris 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
Acked-by: default avatarBorislav Petkov <bp@suse.de>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
parent 85c3fe91
......@@ -193,6 +193,7 @@
#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 */
......
......@@ -91,15 +91,21 @@ struct kvm_vcpu_pv_apf_data {
#ifdef __KERNEL__
#include <asm/processor.h>
#include <asm/alternative.h>
extern void kvmclock_init(void);
extern int kvm_register_clock(char *txt);
/* 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 vmrun or the vmmrun
* instruction. The hypervisor may replace it with something else but only the
......
......@@ -469,6 +469,13 @@ static void __cpuinit early_init_amd(struct cpuinfo_x86 *c)
set_cpu_cap(c, X86_FEATURE_EXTD_APICID);
}
#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);
}
static void __cpuinit init_amd(struct cpuinfo_x86 *c)
......
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