linux-kernel - [RFC PATCH 2/8] kvm: use slowpath in gfn_to_hva

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Message-ID: <20240709132041.3625501-3-roypat@amazon.co.uk>
Date: Tue, 9 Jul 2024 14:20:30 +0100
From: Patrick Roy <roypat@...zon.co.uk>
To: <seanjc@...gle.com>, <pbonzini@...hat.com>, <akpm@...ux-foundation.org>,
	<dwmw@...zon.co.uk>, <rppt@...nel.org>, <david@...hat.com>
CC: Patrick Roy <roypat@...zon.co.uk>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
	<x86@...nel.org>, <hpa@...or.com>, <willy@...radead.org>, <graf@...zon.com>,
	<derekmn@...zon.com>, <kalyazin@...zon.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>, <dmatlack@...gle.com>,
	<tabba@...gle.com>, <chao.p.peng@...ux.intel.com>, <xmarcalx@...zon.co.uk>
Subject: [RFC PATCH 2/8] kvm: use slowpath in gfn_to_hva_cache if memory is private

Currently, KVM uses gfn_to_hva_caches to cache gfn->memslot->userspace
host virtual address (uhva) translations. If a gfn is backed by
guest_memfd however, there is no uhva-equivalent item we could possible
cache, since accesses go through a file descriptor instead of a VMA.
Thus, we effectively disable gfn_to_hva_caches in the case where gfns
are gmem-backed, and instead do a gfn->pfn translation on the fly by
calling `kvm_{read,write}_guest` inside `kvm_{read,write}_guest_cached`.

Signed-off-by: Patrick Roy <roypat@...zon.co.uk>
---
 virt/kvm/kvm_main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b3b3de70a4df..4357f7cdf040 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3603,7 +3603,7 @@ int kvm_write_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 	if (kvm_is_error_hva(ghc->hva))
 		return -EFAULT;
 
-	if (unlikely(!ghc->memslot))
+	if (unlikely(!ghc->memslot || kvm_mem_is_private(kvm, gpa_to_gfn(gpa))))
 		return kvm_write_guest(kvm, gpa, data, len);
 
 	r = __copy_to_user((void __user *)ghc->hva + offset, data, len);
@@ -3641,7 +3641,7 @@ int kvm_read_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 	if (kvm_is_error_hva(ghc->hva))
 		return -EFAULT;
 
-	if (unlikely(!ghc->memslot))
+	if (unlikely(!ghc->memslot || kvm_mem_is_private(kvm, gpa_to_gfn(gpa))))
 		return kvm_read_guest(kvm, gpa, data, len);
 
 	r = __copy_from_user(data, (void __user *)ghc->hva + offset, len);
-- 
2.45.2