Re: KVM: How does is PAT emulation supposed to work?
On 2015-04-17 18:43, Radim Krčmář wrote: 2015-04-13 07:16+0200, Jan Kiszka: PS: If someone has a good idea for a simple test case on machines without IOMMU (like my current boxes), thus without a chance to use device pass-through to stress guest PAT settings, I would be all ears. Not a good one: KVM sets VMX_EPT_IPAT_BIT for RAM unless kvm_arch_has_noncoherent_dma(). You can comment the line in vmx_get_mt_mask(), or call kvm_arch_register_noncoherent_dma(), for guest PAT to work on normal memory. That's for VMX (where I do have IOMMUs), but I would need something for AMD. :) Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: KVM: How does is PAT emulation supposed to work?
2015-04-13 07:16+0200, Jan Kiszka: Hi all, while digging into the PAT topic for Jailhouse, I also wondered how KVM deals with it. And I'm still not getting it complete - or there is a bug: KVM intercepts all guest writes to the PAT MSR and instead keeps the guest value in vcpu-arch.pat. But, besides returning that value back on read accesses, arch.pat has no other purpose. On Intel, we only seem to have proper emulation - through hardware - when VMX supports PAT switching (see vmx_set_msr). On AMD, the situation is even worse as the g_pat save field is not updated at all on PAT writes. That seems to be a low hanging fruit to bring svm on the same support level as vmx. Or am I missing something? I don't think so, it looks buggy ... we could switch PAT manually, if it isn't accelerated by hardware. PS: If someone has a good idea for a simple test case on machines without IOMMU (like my current boxes), thus without a chance to use device pass-through to stress guest PAT settings, I would be all ears. Not a good one: KVM sets VMX_EPT_IPAT_BIT for RAM unless kvm_arch_has_noncoherent_dma(). You can comment the line in vmx_get_mt_mask(), or call kvm_arch_register_noncoherent_dma(), for guest PAT to work on normal memory. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: KVM: How does is PAT emulation supposed to work?
2015-04-17 19:12+0200, Jan Kiszka: On 2015-04-17 18:43, Radim Krčmář wrote: 2015-04-13 07:16+0200, Jan Kiszka: PS: If someone has a good idea for a simple test case on machines without IOMMU (like my current boxes), thus without a chance to use device pass-through to stress guest PAT settings, I would be all ears. Not a good one: KVM sets VMX_EPT_IPAT_BIT for RAM unless kvm_arch_has_noncoherent_dma(). You can comment the line in vmx_get_mt_mask(), or call kvm_arch_register_noncoherent_dma(), for guest PAT to work on normal memory. That's for VMX (where I do have IOMMUs), but I would need something for AMD. :) Ah, your patch should be sufficient on AMD ... (At least the documentation doesn't list other NPT switches.) The simple test case is 'wrmsr(0x277, 0x0707070707070707)' and RAM accesses in a bounded loop -- they aren't significantly slower? -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
KVM: How does is PAT emulation supposed to work?
Hi all, while digging into the PAT topic for Jailhouse, I also wondered how KVM deals with it. And I'm still not getting it complete - or there is a bug: KVM intercepts all guest writes to the PAT MSR and instead keeps the guest value in vcpu-arch.pat. But, besides returning that value back on read accesses, arch.pat has no other purpose. On Intel, we only seem to have proper emulation - through hardware - when VMX supports PAT switching (see vmx_set_msr). On AMD, the situation is even worse as the g_pat save field is not updated at all on PAT writes. That seems to be a low hanging fruit to bring svm on the same support level as vmx. Or am I missing something? Jan PS: If someone has a good idea for a simple test case on machines without IOMMU (like my current boxes), thus without a chance to use device pass-through to stress guest PAT settings, I would be all ears. -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
