linux-kernel - [PATCH] kvm: fix gpu passthrough into vm on arm64

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Message-Id: <20220323012519.521058-1-xieming@kylinos.cn>
Date:   Wed, 23 Mar 2022 09:25:19 +0800
From:   xieming <xieming@...inos.cn>
To:     christoffer.dall@....com, marc.zyngier@....com
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, xieming@...inos.cn
Subject: [PATCH] kvm: fix gpu passthrough into vm on arm64

1) when passthrough some pcie device, such as AMD gpus,
        kvm will report:"Unsupported FSC:" err.

2) the main reason is kvm setting memory type to
   PAGE_S2_DEVICE(DEVICE_nGnRE), but in guestos, all of device io memory
   type when ioremapping (including gpu driver TTM memory type) is
   setting to MT_NORMAL_NC.

3) according to ARM64 stage1&stage2 conbining rules.
   memory type attributes combining rules:
   Normal-WB < Normal-WT <  NormalNC <  Device-GRE <  Device-nGRE <
   DevicenGnRE < Device-nGnRnE
   Normal-WB is weakest,Device-nGnRnE is strongest.

   refferring to 'Arm Architecture Reference Manual Armv8,
   for Armv8-A architecture profile' pdf, chapter B2.8
   refferring to 'ARM System Memory Management Unit Architecture
   Specification SMMU architecture version 3.0 and version 3.1' pdf,
   chapter 13.1.5

4) therefore, the I/O memory attribute of the VM is setting to
   DevicenGnRE is a big mistake. it causes all device memory accessing in
   the virtual machine must be aligned.

   To summarize: stage2 memory type cannot be stronger than stage1 in
   arm64 archtechture.

Signed-off-by: xieming <xieming@...inos.cn>
---
 virt/kvm/arm/mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index 11103b75c596..9b7fb13f4546 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -1209,7 +1209,7 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
 	pfn = __phys_to_pfn(pa);

 	for (addr = guest_ipa; addr < end; addr += PAGE_SIZE) {
-		pte_t pte = pfn_pte(pfn, PAGE_S2_DEVICE);
+		pte_t pte = pfn_pte(pfn, PAGE_S2);

 		if (writable)
 			pte = kvm_s2pte_mkwrite(pte);
-- 
2.27.0