Hi everyone, I am using KVM to run a virtual machine (x86) without having a guest OS. So the machine state is set for long mode before calling KVM_RUN and I am trapping out of KVM on every syscall to emulate the syscall in the program that uses KVM. Also I am trapping out of KVM on pagefaults to deal with stack growth. This has been working great for integer workloads, however I have encoutered some workloads which are using floating point instructions, for example cvtsi2sd to cast an integer to a double, and their result is incorrect. Recently I have discovered that KVM does FPU context save on-demand, meaning when the virtual machine actually uses the FPU [1]. I am wondering if this machanism has some baked in assumptions about running a virtual machine with a guest OS and if a guest OS is not present the FPU state might be altered? Also I have not seen any #NM exceptions, these usually trap out of KVM. The FPU state that I am using [2] seems valid and it does not change when trapping out of KVM for syscalls and pagefaults. In addition, I have tried running with multiple linux versions 2.6.29, 3.13 and 4.0.4 and all of them give incorrect floating point instructions result, however for different instructions. Thank you, -- Alex [1] http://www.linux-kvm.org/images/e/ea/2010-forum-mtosatti_walkthrough_entry_exit.pdf [2] FPU related machine state before calling KVM_RUN: Efer efer = 0; efer.sce = 1; // Enable system call extensions. efer.lme = 1; // Enable long mode. efer.lma = 1; // Activate long mode. efer.nxe = 0; // Enable nx support. efer.svme = 1; // Enable svm support for now. efer.ffxsr = 0; // Turn on fast fxsave and fxrstor. //Set up the registers that describe the operating mode. CR0 cr0 = 0; cr0.pg = 1; // Turn on paging. cr0.cd = 0; // Don't disable caching. cr0.nw = 0; // This is bit is defined to be ignored. cr0.am = 1; // No alignment checking cr0.wp = 1; // Supervisor mode can write read only pages cr0.ne = 1; cr0.et = 1; // This should always be 1 cr0.ts = 0; // We don't do task switching, so causing fp exceptions // would be pointless. cr0.em = 0; // Allow x87 instructions to execute natively. cr0.mp = 1; // This doesn't really matter, but the manual suggests // setting it to one. cr0.pe = 1; // We're definitely in protected mode. CR4 cr4 = 0; //Turn on pae. cr4.osxsave = 1; // Enable XSAVE and Proc Extended States cr4.osxmmexcpt = 1; // Operating System Unmasked Exception cr4.osfxsr = 1; // Operating System FXSave/FSRSTOR Support cr4.pce = 0; // Performance-Monitoring Counter Enable cr4.pge = 0; // Page-Global Enable cr4.mce = 0; // Machine Check Enable cr4.pae = 1; // Physical-Address Extension cr4.pse = 0; // Page Size Extensions cr4.de = 0; // Debugging Extensions cr4.tsd = 0; // Time Stamp Disable cr4.pvi = 0; // Protected-Mode Virtual Interrupts cr4.vme = 0; // Virtual-8086 Mode Extensions -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
