[kvmtool PATCH v2 1/1] riscv: Fix the hart bit setting of AIA - Yong-Xuan Wang

From: Yong-Xuan Wang <yongxuan.wang@sifive.com>
To: linux-riscv@lists.infradead.org, kvm-riscv@lists.infradead.org,
	kvm@vger.kernel.org
Cc: will@kernel.org, julien.thierry.kdev@gmail.com,
	apatel@ventanamicro.com, greentime.hu@sifive.com,
	vincent.chen@sifive.com,
	Yong-Xuan Wang <yongxuan.wang@sifive.com>
Subject: [kvmtool PATCH v2 1/1] riscv: Fix the hart bit setting of AIA
Date: Wed, 15 May 2024 17:19:02 +0800	[thread overview]
Message-ID: <20240515091902.28368-1-yongxuan.wang@sifive.com> (raw)

In AIA spec, each hart (or each hart within a group) has a unique hart
number to locate the memory pages of interrupt files in the address
space. The number of bits required to represent any hart number is equal
to ceil(log2(hmax + 1)), where hmax is the largest hart number among
groups.

However, if the largest hart number among groups is a power of 2, QEMU
will pass an inaccurate hart-index-bit setting to Linux. For example, when
the guest OS has 4 harts, only ceil(log2(3 + 1)) = 2 bits are sufficient
to represent 4 harts, but we passes 3 to Linux. The code needs to be
updated to ensure accurate hart-index-bit settings.

Additionally, a Linux patch[1] is necessary to correctly recover the hart
index when the guest OS has only 1 hart, where the hart-index-bit is 0.

[1] https://lore.kernel.org/lkml/20240415064905.25184-1-yongxuan.wang@sifive.com/t/

Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
---
Changelog
v2:
- update commit message
---
 riscv/aia.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/riscv/aia.c b/riscv/aia.c
index fe9399a8ffc1..21d9704145d0 100644
--- a/riscv/aia.c
+++ b/riscv/aia.c
@@ -164,7 +164,7 @@ static int aia__init(struct kvm *kvm)
 	ret = ioctl(aia_fd, KVM_SET_DEVICE_ATTR, &aia_nr_sources_attr);
 	if (ret)
 		return ret;
-	aia_hart_bits = fls_long(kvm->nrcpus);
+	aia_hart_bits = fls_long(kvm->nrcpus - 1);
 	ret = ioctl(aia_fd, KVM_SET_DEVICE_ATTR, &aia_hart_bits_attr);
 	if (ret)
 		return ret;
-- 
2.17.1


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