[PATCH 1/2] KVM: x86: Fix poll command

[PATCH 1/2] KVM: x86: Fix poll command

[alexjlzheng]

From: Jinliang Zheng <alexjlzheng@tencent.com>

According to the hardware manual, when the Poll command is issued, the
byte returned by the I/O read is 1 in Bit 7 when there is an interrupt,
and the highest priority binary code in Bits 2:0. The current pic
simulation code is not implemented strictly according to the above
expression.

Fix the implementation of pic_poll_read(), set Bit 7 when there is an
interrupt.

Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
---
 arch/x86/kvm/i8259.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/i8259.c b/arch/x86/kvm/i8259.c
index 4756bcb5724f..861872e2641a 100644
--- a/arch/x86/kvm/i8259.c
+++ b/arch/x86/kvm/i8259.c
@@ -411,6 +411,8 @@ static u32 pic_poll_read(struct kvm_kpic_state *s, u32 addr1)
 		pic_clear_isr(s, ret);
 		if (addr1 >> 7 || ret != 2)
 			pic_update_irq(s->pics_state);
+		/* Bit 7 is 1, means there's an interrupt */
+		ret |= 0x80;
 	} else {
 		ret = 0x07;
 		pic_update_irq(s->pics_state);
-- 
2.31.1

[PATCH 2/2] KVM: x86: Adjust return value of pic_poll_read()

[alexjlzheng]

From: Jinliang Zheng <alexjlzheng@tencent.com>

Returning 0x07 raises ambiguity when no interrupt is in pic_poll_read().
Although it will not cause a functional exception (Bit 7 is 0 means no
interrupt), it will easily make developers mistakenly think that a
spurious interrupt (IRQ 7) has been returned.

Return 0x00 instread of 0x07.

Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
---
 arch/x86/kvm/i8259.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/i8259.c b/arch/x86/kvm/i8259.c
index 861872e2641a..57978ad8311c 100644
--- a/arch/x86/kvm/i8259.c
+++ b/arch/x86/kvm/i8259.c
@@ -414,7 +414,8 @@ static u32 pic_poll_read(struct kvm_kpic_state *s, u32 addr1)
 		/* Bit 7 is 1, means there's an interrupt */
 		ret |= 0x80;
 	} else {
-		ret = 0x07;
+		/* Bit 7 is 0, means there's no interrupt */
+		ret = 0x00;
 		pic_update_irq(s->pics_state);
 	}
 
-- 
2.31.1

Re: [PATCH 2/2] KVM: x86: Adjust return value of pic_poll_read()

[Sean Christopherson]

On Tue, Apr 18, 2023, alexjlzheng@gmail.com wrote:
> From: Jinliang Zheng <alexjlzheng@tencent.com>
> 
> Returning 0x07 raises ambiguity when no interrupt is in pic_poll_read().
> Although it will not cause a functional exception (Bit 7 is 0 means no

From KVM's perspective, it's a functional change.  It _shouldn't_ impact the
overall functionality of the guest, but we have no idea what guest code exists
in the wild.

> interrupt), it will easily make developers mistakenly think that a
> spurious interrupt (IRQ 7) has been returned.
> 
> Return 0x00 instread of 0x07.

Again, I do not want to introduce a functional change in this code without evidence
that the change fixes something for a real world guest.  Based on your response[*],
that is not the case.

A comment explaining the KVM behavior would be very welcome, but I'm not taking
this patch.

[*] https://lore.kernel.org/all/20230418075923.752113-1-alexjlzheng@tencent.com