Re: [PATCH 2/4] KVM: x86/mmu: Fully re-evaluate MMIO caching when SPTE masks change

[Kai Huang]

On Thu, 2022-07-28 at 22:17 +0000, Sean Christopherson wrote:
> Fully re-evaluate whether or not MMIO caching can be enabled when SPTE
> masks change; simply clearing enable_mmio_caching when a configuration
> isn't compatible with caching fails to handle the scenario where the
> masks are updated, e.g. by VMX for EPT or by SVM to account for the C-bit
> location, and toggle compatibility from false=>true.
> 
> Snapshot the original module param so that re-evaluating MMIO caching
> preserves userspace's desire to allow caching.  Use a snapshot approach
> so that enable_mmio_caching still reflects KVM's actual behavior.
> 
> Fixes: 8b9e74bfbf8c ("KVM: x86/mmu: Use enable_mmio_caching to track if MMIO caching is enabled")
> Reported-by: Michael Roth <michael.roth@xxxxxxx>
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c  |  4 ++++
>  arch/x86/kvm/mmu/spte.c | 19 +++++++++++++++++++
>  arch/x86/kvm/mmu/spte.h |  1 +
>  3 files changed, 24 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 2975fcb14c86..660f58928252 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6699,11 +6699,15 @@ static int set_nx_huge_pages(const char *val, const struct kernel_param *kp)
>  /*
>   * nx_huge_pages needs to be resolved to true/false when kvm.ko is loaded, as
>   * its default value of -1 is technically undefined behavior for a boolean.
> + * Forward the module init call to SPTE code so that it too can handle module
> + * params that need to be resolved/snapshot.
>   */
>  void __init kvm_mmu_x86_module_init(void)
>  {
>  	if (nx_huge_pages == -1)
>  		__set_nx_huge_pages(get_nx_auto_mode());
> +
> +	kvm_mmu_spte_module_init();
>  }
>  
>  /*
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 7314d27d57a4..66f76f5a15bd 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -20,6 +20,7 @@
>  #include <asm/vmx.h>
>  
>  bool __read_mostly enable_mmio_caching = true;
> +static bool __ro_after_init allow_mmio_caching;
>  module_param_named(mmio_caching, enable_mmio_caching, bool, 0444);
>  
>  u64 __read_mostly shadow_host_writable_mask;
> @@ -43,6 +44,18 @@ u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;
>  
>  u8 __read_mostly shadow_phys_bits;
>  
> +void __init kvm_mmu_spte_module_init(void)
> +{
> +	/*
> +	 * Snapshot userspace's desire to allow MMIO caching.  Whether or not
> +	 * KVM can actually enable MMIO caching depends on vendor-specific
> +	 * hardware capabilities and other module params that can't be resolved
> +	 * until the vendor module is loaded, i.e. enable_mmio_caching can and
> +	 * will change when the vendor module is (re)loaded.
> +	 */
> +	allow_mmio_caching = enable_mmio_caching;
> +}
> +
>  static u64 generation_mmio_spte_mask(u64 gen)
>  {
>  	u64 mask;
> @@ -340,6 +353,12 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
>  	BUG_ON((u64)(unsigned)access_mask != access_mask);
>  	WARN_ON(mmio_value & shadow_nonpresent_or_rsvd_lower_gfn_mask);
>  
> +	/*
> +	 * Reset to the original module param value to honor userspace's desire
> +	 * to (dis)allow MMIO caching.  Update the param itself so that
> +	 * userspace can see whether or not KVM is actually using MMIO caching.
> +	 */
> +	enable_mmio_caching = allow_mmio_caching;

I think the problem comes from MMIO caching mask/value are firstly set in
kvm_mmu_reset_all_pte_masks() (which calls kvm_mmu_set_mmio_spte_mask() and may
change enable_mmio_caching), and later vendor specific code _may_ or _may_not_
call kvm_mmu_set_mmio_spte_mask() again to adjust the mask/value.  And when it
does, the second call from vendor specific code shouldn't depend on the
'enable_mmio_caching' value calculated in the first call in 
kvm_mmu_reset_all_pte_masks().

Instead of using 'allow_mmio_caching', should we just remove
kvm_mmu_set_mmio_spte_mask() in kvm_mmu_reset_all_pte_masks() and enforce vendor
specific code to always call kvm_mmu_set_mmio_spte_mask() depending on whatever
hardware feature the vendor uses?

I am suggesting this way because in Isaku's TDX patch

[PATCH v7 037/102] KVM: x86/mmu: Track shadow MMIO value/mask on a per-VM basis

we will enable per-VM MMIO mask/value, which will remove global
shadow_mmio_mask/shadow_mmio_value, and I am already suggesting something
similar:

https://lore.kernel.org/all/20220719084737.GU1379820@xxxxxxxxxxxxxxxxxxxxx/