Re: [PATCH] KVM: x86/mmu: Allocate PAE root array when using SVM's 32-bit NPT

[Jiri Palecek <jpalecek@web.de>]

Hello,

On 13. 06. 19 19:22, Sean Christopherson wrote:
> SVM's Nested Page Tables (NPT) reuses x86 paging for the host-controlled
> page walk.  For 32-bit KVM, this means PAE paging is used even when TDP
> is enabled, i.e. the PAE root array needs to be allocated.
>
> Fixes: ee6268ba3a68 ("KVM: x86: Skip pae_root shadow allocation if tdp enabled")
> Cc: stable@vger.kernel.org
> Reported-by: Jiri Palecek <jpalecek@web.de>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>
> Jiri, can you please test this patch?  I haven't actually verified this
> fixes the bug due to lack of SVM hardware.

Yes, it makes KVM work again on i686 pae and nopae with kernel 5.2-rc4.
However, I also tried, just out of curiosity, a nested kvm setup and got
a kvm_spurious_fault in L1 every time. I'll probably file another bug
for that.

Thanks for your involvement

     Jiri Palecek


>
>   arch/x86/kvm/mmu.c | 16 ++++++++++------
>   1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 1e9ba81accba..d3c3d5e5ffd4 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -5602,14 +5602,18 @@ static int alloc_mmu_pages(struct kvm_vcpu *vcpu)
>   	struct page *page;
>   	int i;
>
> -	if (tdp_enabled)
> -		return 0;
> -
>   	/*
> -	 * When emulating 32-bit mode, cr3 is only 32 bits even on x86_64.
> -	 * Therefore we need to allocate shadow page tables in the first
> -	 * 4GB of memory, which happens to fit the DMA32 zone.
> +	 * When using PAE paging, the four PDPTEs are treated as 'root' pages,
> +	 * while the PDP table is a per-vCPU construct that's allocated at MMU
> +	 * creation.  When emulating 32-bit mode, cr3 is only 32 bits even on
> +	 * x86_64.  Therefore we need to allocate the PDP table in the first
> +	 * 4GB of memory, which happens to fit the DMA32 zone.  Except for
> +	 * SVM's 32-bit NPT support, TDP paging doesn't use PAE paging and can
> +	 * skip allocating the PDP table.
>   	 */
> +	if (tdp_enabled && kvm_x86_ops->get_tdp_level(vcpu) > PT32E_ROOT_LEVEL)
> +		return 0;
> +
>   	page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_DMA32);
>   	if (!page)
>   		return -ENOMEM;