linux-kernel - Re: [PATCH v4 21/21] KVM: selftests: Test READ=>WRITE dirty logging behavior for shadow MMU

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Message-ID: <kivowzryb3mbntaoud4kmhxjbix3usqjiy7phwihjc7p4rp4gk@ppilefrxpu35>
Date: Thu, 8 Jan 2026 20:26:07 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oupton@...nel.org>, Tianrui Zhao <zhaotianrui@...ngson.cn>, 
	Bibo Mao <maobibo@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>, 
	Anup Patel <anup@...infault.org>, Paul Walmsley <pjw@...nel.org>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Christian Borntraeger <borntraeger@...ux.ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, 
	Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, loongarch@...ts.linux.dev, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 21/21] KVM: selftests: Test READ=>WRITE dirty logging
 behavior for shadow MMU

On Thu, Jan 08, 2026 at 08:32:44AM -0800, Sean Christopherson wrote:
[..]
> @@ -106,12 +139,66 @@ static void l1_guest_code(void *data)
>  		l1_svm_code(data);
>  }
>  
> +static void test_handle_ucall_sync(struct kvm_vm *vm, u64 arg,
> +				   unsigned long *bmap)
> +{
> +	vm_vaddr_t gva = arg & ~(PAGE_SIZE - 1);
> +	int page_nr, i;
> +
> +	/*
> +	 * Extract the page number of underlying physical page, which is also
> +	 * the _L1_ page number.  The dirty bitmap _must_ be updated based on
> +	 * the L1 GPA, not L2 GPA, i.e. whether or not L2 used an aliased GPA
> +	 * (i.e. if TDP enabled for L2) is irrelevant with respect to the dirty
> +	 * bitmap and which underlying physical page is accessed.
> +	 *
> +	 * Note, gva will be '0' if there was no access, i.e. if the purpose of
> +	 * the sync is to verify all pages are clean.
> +	 */
> +	if (!gva)
> +		page_nr = 0;
> +	else if (gva >= TEST_MEM_ALIAS_BASE)
> +		page_nr = (gva - TEST_MEM_ALIAS_BASE) >> PAGE_SHIFT;
> +	else
> +		page_nr = (gva - TEST_MEM_BASE) >> PAGE_SHIFT;
> +	TEST_ASSERT(page_nr == 0 || page_nr == 1,
> +		    "Test bug, unexpected frame number '%u' for arg = %lx", page_nr, arg);
> +	TEST_ASSERT(gva || (arg & TEST_SYNC_NO_FAULT),
> +		    "Test bug, gva must be valid if a fault is expected");
> +
> +	kvm_vm_get_dirty_log(vm, TEST_MEM_SLOT_INDEX, bmap);
> +
> +	/*
> +	 * Check all pages to verify the correct physical page was modified (or
> +	 * not), and that all pages are clean/dirty as expected.
> +	 *
> +	 * If a fault of any kind is expected, the target page should be dirty
> +	 * as the Dirty bit is set in the gPTE.  KVM should create a writable
> +	 * SPTE even on a read fault, *and* KVM must mark the GFN as dirty
> +	 * when doing so.
> +	 */
> +	for (i = 0; i < TEST_MEM_PAGES; i++) {
> +		if (i == page_nr && arg & TEST_SYNC_WRITE_FAULT)

Micro nit: I think this is slightly clearer:
		if (i == page_nr && (arg & TEST_SYNC_WRITE_FAULT))