Re: [PATCH RFC 4/5] KVM: x86: aggressively map PTEs in KVM_MEM_ALLONES slots

[Sean Christopherson]

On Thu, May 14, 2020 at 08:05:39PM +0200, Vitaly Kuznetsov wrote:
> All PTEs in KVM_MEM_ALLONES slots point to the same read-only page
> in KVM so instead of mapping each page upon first access we can map
> everything aggressively.
> 
> Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c         | 20 ++++++++++++++++++--
>  arch/x86/kvm/mmu/paging_tmpl.h | 23 +++++++++++++++++++++--
>  2 files changed, 39 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 3db499df2dfc..e92ca9ed3ff5 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4154,8 +4154,24 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
>  		goto out_unlock;
>  	if (make_mmu_pages_available(vcpu) < 0)
>  		goto out_unlock;
> -	r = __direct_map(vcpu, gpa, write, map_writable, max_level, pfn,
> -			 prefault, is_tdp && lpage_disallowed);
> +
> +	if (likely(!(slot->flags & KVM_MEM_ALLONES) || write)) {

The 'write' check is wrong.  More specifically, patch 2/5 is missing code
to add KVM_MEM_ALLONES to memslot_is_readonly().  If we end up going with
an actual kvm_allones_pg backing, writes to an ALLONES memslots should be
handled same as writes to RO memslots; MMIO occurs but no MMIO spte is
created.

> +		r = __direct_map(vcpu, gpa, write, map_writable, max_level, pfn,
> +				 prefault, is_tdp && lpage_disallowed);
> +	} else {
> +		/*
> +		 * KVM_MEM_ALLONES are 4k only slots fully mapped to the same
> +		 * readonly 'allones' page, map all PTEs aggressively here.
> +		 */
> +		for (gfn = slot->base_gfn; gfn < slot->base_gfn + slot->npages;
> +		     gfn++) {
> +			r = __direct_map(vcpu, gfn << PAGE_SHIFT, write,
> +					 map_writable, max_level, pfn, prefault,
> +					 is_tdp && lpage_disallowed);

IMO this is a waste of memory and TLB entries.  Why not treat the access as
the MMIO it is and emulate the access with a 0xff return value?  I think
it'd be a simple change to have __kvm_read_guest_page() stuff 0xff, i.e. a
kvm_allones_pg wouldn't be needed.  I would even vote to never create an
MMIO SPTE.  The guest has bigger issues if reading from a PCI hole is
performance sensitive.

Regarding memory, looping wantonly on __direct_map() will eventually trigger
the BUG_ON() in mmu_memory_cache_alloc().  mmu_topup_memory_caches() only
ensures there are enough objects available to map a single translation, i.e.
one entry per level, sans the root[*].

[*] The gorilla math in mmu_topup_memory_caches() is horrendously misleading,
    e.g. the '8' pages is really 2*(ROOT_LEVEL - 1), but the 2x part has been
    obsolete for the better part of a decade, and the '- 1' wasn't actually
    originally intended or needed, but is now required because of 5-level
    paging.  I have the beginning of a series to clean up that mess; it was
    low on my todo list because I didn't expect anyone to be mucking with
    related code :-)

> +			if (r)
> +				break;
> +		}
> +	}