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Re: [PATCH v2] x86/svm: Intercept CR0 writes selectively
- To: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 8 Sep 2026 10:44:10 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Jason Andryuk <jason.andryuk@xxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 08 Sep 2026 08:44:21 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 04.09.2026 14:23, Ross Lagerwall wrote:
> @@ -2517,6 +2518,7 @@ void asmlinkage svm_vmexit_handler(void)
> hvm_sanitize_regs_fields(
> regs, !(vmcb_get_efer(vmcb) & EFER_LMA) || !(vmcb->cs.l));
>
> + v->arch.hvm.guest_cr[0] = vmcb_get_cr0(vmcb);
> v->arch.hvm.guest_cr[2] = vmcb_get_cr2(vmcb);
> if ( paging_mode_hap(v->domain) )
> v->arch.hvm.guest_cr[3] = v->arch.hvm.hw_cr[3] = vmcb_get_cr3(vmcb);
gitlab-CI says no to this patch [1], and I think it's the change above which
gets in the way of running guests in shadow mode (as is the case for the two
qemu-smoke-x86_64-*-pvh tests). svm_update_guest_cr() has
value = v->arch.hvm.guest_cr[0];
if ( paging_mode_shadow(v->domain) )
value |= X86_CR0_PG | X86_CR0_WP;
vmcb_set_cr0(vmcb, value);
i.e. upon reading back the guest's CR0 value we would wrongly record it
having PG (and WP) enabled, at which point shadow code would try to use the
still-zero CR3 as page table base. I think values need merging here, with
only TS and MP taken from what vmcb_get_cr0() returns.
The zen2-* failure in [1] is, I think, unrelated.
Jan
[1] https://gitlab.com/xen-project/hardware/xen-staging/-/pipelines/2828369624
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