On 2014-07-07 12:38, Alok Kataria wrote:
> From: Jim Mattson <jmattson@xxxxxxxxxx>
>
> We have noticed that qemu-kvm hangs early in the BIOS when runnning nested
> under some versions of VMware ESXi.
>
> The problem we believe is because KVM assumes that the platform preserves
> the 'G' but for any segment register. The SVM specification itemizes the
> segment attribute bits that are observed by the CPU, but the (G)ranularity bit
> is not one of the bits itemized, for any segment. Though current AMD CPUs keep
> track of the (G)ranularity bit for all segment registers other than CS, the
> specification does not require it. VMware's virtual CPU may not track the
> (G)ranularity bit for any segment register.
>
> Since kvm already synthesizes the (G)ranularity bit for the CS segment. It
> should do so for all segments. The patch below does that, and helps get rid of
> the hangs. Patch applies on top of Linus' tree.
>
> Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
> Signed-off-by: Alok N Kataria <akataria@xxxxxxxxxx>
>
> Index: linux-2.6/arch/x86/kvm/svm.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kvm/svm.c 2014-07-07 15:32:52.724368183 +0530
> +++ linux-2.6/arch/x86/kvm/svm.c 2014-07-07 15:34:19.664748841 +0530
> @@ -1415,7 +1415,7 @@
> var->avl = (s->attrib >> SVM_SELECTOR_AVL_SHIFT) & 1;
> var->l = (s->attrib >> SVM_SELECTOR_L_SHIFT) & 1;
> var->db = (s->attrib >> SVM_SELECTOR_DB_SHIFT) & 1;
> - var->g = (s->attrib >> SVM_SELECTOR_G_SHIFT) & 1;
> + var->g = s->limit > 0xfffff;
>
> /*
> * AMD's VMCB does not have an explicit unusable field, so emulate it
> @@ -1424,14 +1424,6 @@
> var->unusable = !var->present || (var->type == 0);
>
> switch (seg) {
> - case VCPU_SREG_CS:
> - /*
> - * SVM always stores 0 for the 'G' bit in the CS selector in
> - * the VMCB on a VMEXIT. This hurts cross-vendor migration:
> - * Intel's VMENTRY has a check on the 'G' bit.
> - */
> - var->g = s->limit > 0xfffff;
> - break;
> case VCPU_SREG_TR:
> /*
> * Work around a bug where the busy flag in the tr selector
>
>
Thanks for pushing this. I already tried to analyze the spec in this
regard in [1].
But even if it turns out we could read the bit on real HW, I think this
patch is fine in order to be compatible with ESXi.
Jan
[1] http://thread.gmane.org/gmane.comp.emulators.kvm.devel/124252
--
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 majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
