Re: [PATCH] KVM: selftests: Create memslot 0 at GPA 0x100000000 on x86_64

From: Sean Christopherson <seanjc@google.com>
To: David Matlack <dmatlack@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Fuad Tabba <tabba@google.com>,  Peter Gonda <pgonda@google.com>,
	Ackerley Tng <ackerleytng@google.com>,
	 Chao Peng <chao.p.peng@linux.intel.com>,
	Vishal Annapurve <vannapurve@google.com>,
	 Michael Roth <michael.roth@amd.com>,
	Andrew Jones <ajones@ventanamicro.com>,
	kvm@vger.kernel.org
Subject: Re: [PATCH] KVM: selftests: Create memslot 0 at GPA 0x100000000 on x86_64
Date: Thu, 7 Mar 2024 14:37:09 -0800	[thread overview]
Message-ID: <ZepBlYLPSuhISTTc@google.com> (raw)
In-Reply-To: <20240307194255.1367442-1-dmatlack@google.com>

On Thu, Mar 07, 2024, David Matlack wrote:
> Create memslot 0 at 0x100000000 (4GiB) to avoid it overlapping with
> KVM's private memslot for the APIC-access page.

This is going to cause other problems, e.g. from max_guest_memory_test.c

	/*
	 * Skip the first 4gb and slot0.  slot0 maps <1gb and is used to back
	 * the guest's code, stack, and page tables.  Because selftests creates
	 * an IRQCHIP, a.k.a. a local APIC, KVM creates an internal memslot
	 * just below the 4gb boundary.  This test could create memory at
	 * 1gb-3gb,but it's simpler to skip straight to 4gb.
	 */
	const uint64_t start_gpa = SZ_4G;

Trying to move away from starting at '0' is going to be problematic/annoying,
e.g. using low memory allows tests to safely assume 4GiB+ is always available.
And I'd prefer not to make the infrastucture all twisty and weird for all tests
just because memstress tests want to play with huge amounts of memory.

Any chance we can solve this by using huge pages in the guest, and adjusting the
gorilla math in vm_nr_pages_required() accordingly?  There's really no reason to
use 4KiB pages for a VM with 256GiB of memory.  That'd also be more represantitive
of real world workloads (at least, I hope real world workloads are using 2MiB or
1GiB pages in this case).