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SubjectRe: [PATCH v3] KVM: SVM: Override default MMIO mask if memory encryption is enabled
On Wed, Jan 08, 2020 at 12:40:16PM -0600, Tom Lendacky wrote:
> The KVM MMIO support uses bit 51 as the reserved bit to cause nested page
> faults when a guest performs MMIO. The AMD memory encryption support uses
> a CPUID function to define the encryption bit position. Given this, it is
> possible that these bits can conflict.
>
> Use svm_hardware_setup() to override the MMIO mask if memory encryption
> support is enabled. Various checks are performed to ensure that the mask
> is properly defined and rsvd_bits() is used to generate the new mask (as
> was done prior to the change that necessitated this patch).
>
> Fixes: 28a1f3ac1d0c ("kvm: x86: Set highest physical address bits in non-present/reserved SPTEs")
> Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>

A few nits below, other than that:

Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>

>
> ---
>
> Changes in v3:
> - Add additional checks to ensure there are no conflicts between the
> encryption bit position and physical address setting.
> - Use rsvd_bits() generated mask (as was previously used) instead of
> setting a single bit.
>
> Changes in v2:
> - Use of svm_hardware_setup() to override MMIO mask rather than adding an
> override callback routine.
> ---
> arch/x86/kvm/svm.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 51 insertions(+)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 122d4ce3b1ab..9d6bd3fc12c8 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1307,6 +1307,55 @@ static void shrink_ple_window(struct kvm_vcpu *vcpu)
> }
> }
>
> +/*
> + * The default MMIO mask is a single bit (excluding the present bit),
> + * which could conflict with the memory encryption bit. Check for
> + * memory encryption support and override the default MMIO masks if
> + * it is enabled.
> + */
> +static __init void svm_adjust_mmio_mask(void)
> +{
> + unsigned int enc_bit, mask_bit;
> + u64 msr, mask;
> +
> + /* If there is no memory encryption support, use existing mask */
> + if (cpuid_eax(0x80000000) < 0x8000001f)
> + return;
> +
> + /* If memory encryption is not enabled, use existing mask */
> + rdmsrl(MSR_K8_SYSCFG, msr);
> + if (!(msr & MSR_K8_SYSCFG_MEM_ENCRYPT))
> + return;
> +
> + enc_bit = cpuid_ebx(0x8000001f) & 0x3f;
> + mask_bit = boot_cpu_data.x86_phys_bits;
> +
> + /* Increment the mask bit if it is the same as the encryption bit */
> + if (enc_bit == mask_bit)
> + mask_bit++;

Nice!

> +
> + if (mask_bit > 51) {
> + /*
> + * The mask bit is above 51, so use bit 51 without the present
> + * bit.
> + */
> + mask = BIT_ULL(51);

I don't think setting bit 51 is necessary. Setting a reserved PA bit is
purely to trigger the #PF, the MMIO spte itself is confirmed by the presence
of SPTE_MMIO_MASK.

AFAICT, clearing only the present bit in kvm_set_mmio_spte_mask() is an
odd implementation quirk, i.e. it can, and arguably should, simply clear
the mask. It's something I'd like to clean up (in mmu.c) and would prefer
to not propagate here.

> + } else {
> + /*
> + * Some bits above the physical addressing limit will always
> + * be reserved, so use the rsvd_bits() function to generate
> + * the mask. This mask, along with the present bit, will be
> + * used to generate a page fault with PFER.RSV = 1.
> + */
> + mask = rsvd_bits(mask_bit, 51);
> + mask |= BIT_ULL(0);

My personal preference would be to use PT_PRESENT_MASK (more crud in mmu.c
that should be fixed). And the brackets can be dropped if mask is set in
a single line, e.g.:

/*
* Here be a comment.
*/
if (mask_bit > 51)
mask = 0;
else
mask = rsvd_bits(mask_bit, 51) | PT_PRESENT_MASK;

> + }
> +
> + kvm_mmu_set_mmio_spte_mask(mask, mask,
> + PT_WRITABLE_MASK |
> + PT_USER_MASK);
> +}
> +
> static __init int svm_hardware_setup(void)
> {
> int cpu;
> @@ -1361,6 +1410,8 @@ static __init int svm_hardware_setup(void)
> }
> }
>
> + svm_adjust_mmio_mask();
> +
> for_each_possible_cpu(cpu) {
> r = svm_cpu_init(cpu);
> if (r)
> --
> 2.17.1
>

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