On 2012-11-04 13:15, Hao, Xudong wrote: >> -----Original Message----- >> From: Jan Kiszka [mailto:jan.kiszka@xxxxxx] >> Sent: Saturday, November 03, 2012 6:55 PM >> To: Hao, Xudong >> Cc: qemu-devel@xxxxxxxxxx; avi@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx >> Subject: Re: [PATCH 1/2] qemu-kvm/cpuid: fix a emulation of guest physical >> address space >> >> On 2012-11-02 06:38, Xudong Hao wrote: >>> For 64 bit processor, emulate 40 bits physical address if the host physical >>> address space >= 40bits, else guest physical is same as host. >>> >>> Signed-off-by: Xudong Hao <xudong.hao@xxxxxxxxx> >>> --- >>> target-i386/cpu.c | 5 ++++- >>> 1 files changed, 4 insertions(+), 1 deletions(-) >>> >>> diff --git a/target-i386/cpu.c b/target-i386/cpu.c >>> index 423e009..3a78881 100644 >>> --- a/target-i386/cpu.c >>> +++ b/target-i386/cpu.c >>> @@ -1584,7 +1584,10 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t >> index, uint32_t count, >>> if (env->cpuid_ext2_features & CPUID_EXT2_LM) { >>> /* 64 bit processor */ >>> /* XXX: The physical address space is limited to 42 bits in exec.c. */ >>> - *eax = 0x00003028; /* 48 bits virtual, 40 bits physical */ >>> +/* XXX: 40 bits physical if host physical address space >= 40 bits */ >>> + uint32_t a, b, c, d; >>> + host_cpuid(0x80000008, 0, &a, &b, &c, &d); >>> + *eax = a < 0x00003028 ? a : 0x00003028; >> >> This variation will not only affect -cpu host, right? That can create >> problems when migrating between hosts with different address widths, and >> then we will need some control knob to adjust what it reported to the guest. >> > > Oh, I did not consider migrating to different platform(addr widths). > But I think the fixed value 40 bits may cause problem: in VT-d case, when a host support GAW < 40 bits, and qemu emulate 40 bits guest physical address space, will bring bug on: > > drivers/iommu/intel-iommu.c > static struct dma_pte *pfn_to_dma_pte(struct dmar_domain *domain, > unsigned long pfn, int target_level) > { > int addr_width = agaw_to_width(domain->agaw) - VTD_PAGE_SHIFT; > ... > BUG_ON(!domain->pgd); > BUG_ON(addr_width < BITS_PER_LONG && pfn >> addr_width); > Does it mean that buggy or malicious user space can trigger a kernel bug? Then this must be fixed of course. Jan
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