[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [PATCH v2 1/2][4.15] x86/PV: conditionally avoid raising #GP for early guest MSR reads
Prior to 4.15 Linux, when running in PV mode, did not install a #GP handler early enough to cover for example the rdmsrl_safe() of MSR_K8_TSEG_ADDR in bsp_init_amd() (not to speak of the unguarded read of MSR_K7_HWCR later in the same function). The respective change (42b3a4cb5609 "x86/xen: Support early interrupts in xen pv guests") was backported to 4.14, but no further - presumably since it wasn't really easy because of other dependencies. Therefore, to prevent our change in the handling of guest MSR accesses to render PV Linux 4.13 and older unusable on at least AMD systems, make the raising of #GP on this paths conditional upon the guest having installed a handler, provided of course the MSR can be read in the first place (we would have raised #GP in that case even before). Producing zero for reads isn't necessarily correct and may trip code trying to detect presence of MSRs early, but since such detection logic won't work without a #GP handler anyway, this ought to be a fair workaround. Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx> --- v2: Probe MSR read. Exclude hypervisor range. Avoid issuing two log messages (in debug builds). Don't alter WRMSR behavior. --- I'm not convinced we can get away without also making the WRMSR path somewhat more permissive again, e.g. tolerating attempts to set bits which are already set. But of course this would require keeping in sync for which MSRs we "fake" reads, as then a kernel attempt to set a bit may also appear as an attempt to clear others (because of the zero value that we gave it for the read). --- a/xen/arch/x86/pv/emul-priv-op.c +++ b/xen/arch/x86/pv/emul-priv-op.c @@ -874,7 +874,7 @@ static int read_msr(unsigned int reg, ui struct vcpu *curr = current; const struct domain *currd = curr->domain; const struct cpuid_policy *cp = currd->arch.cpuid; - bool vpmu_msr = false; + bool vpmu_msr = false, warn = false; int ret; if ( (ret = guest_rdmsr(curr, reg, val)) != X86EMUL_UNHANDLEABLE ) @@ -882,7 +882,7 @@ static int read_msr(unsigned int reg, ui if ( ret == X86EMUL_EXCEPTION ) x86_emul_hw_exception(TRAP_gp_fault, 0, ctxt); - return ret; + goto done; } switch ( reg ) @@ -986,7 +986,7 @@ static int read_msr(unsigned int reg, ui } /* fall through */ default: - gdprintk(XENLOG_WARNING, "RDMSR 0x%08x unimplemented\n", reg); + warn = true; break; normal: @@ -995,7 +995,19 @@ static int read_msr(unsigned int reg, ui return X86EMUL_OKAY; } - return X86EMUL_UNHANDLEABLE; + done: + if ( ret != X86EMUL_OKAY && !curr->arch.pv.trap_ctxt[X86_EXC_GP].address && + (reg >> 16) != 0x4000 && !rdmsr_safe(reg, *val) ) + { + gprintk(XENLOG_WARNING, "faking RDMSR 0x%08x\n", reg); + *val = 0; + x86_emul_reset_event(ctxt); + ret = X86EMUL_OKAY; + } + else if ( warn ) + gdprintk(XENLOG_WARNING, "RDMSR 0x%08x unimplemented\n", reg); + + return ret; } static int write_msr(unsigned int reg, uint64_t val,
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