Re: [BUG?] can not writeback more then 8 bytes memory

[Wei Yongjun <yjwei@cn.fujitsu.com>]

>  On 08/20/2010 12:12 PM, Wei Yongjun wrote:
>   
>> Hi all:
>>
>> I have the following patch to SIDT emulation instruction, but it
>> does not work because we can not writeback more then 8 bytes
>> memory, the SIDT under PROTO64 is 10 bytes.
>>
>> I change the code to write twice, first time write limit, then write
>> address, and it does not work too. If I change the c->dst.bytes
>> to only writeback 8 bytes, it will writeback 8 bytes.
>>
>> Is this a bug of KVM or it is the IO limit?
>>     
> It's complicated... if you write to real memory, then you can do
>
> c->dst.* = ...;
> writeback();
> c->dst.* = ...;
> writeback();
>
> However that will not work for mmio, since writeback() only schedules
> mmio writes which are retired later on.
>
> As a temporary fix we can extend writeback() to support 2+N byte writes
> (and fail them on mmio). See also the sse-mmio branch on kvm.git on how
> to support >8 byte mmio.
>
> Longer term I'd like a queue based approach so we can have any number of
> reads and writes for an instruction, and have multiple writes possible
> for mmio. It would look something like this:
>
> - every read or write is assigned a position in the queue starting from 0
> - the queue is maintained across invocations of x86_emulate_insn()
> - if an operation position is already in the queue, we read the value
> from the queue, and ignore writes (e.g. a replay after exit to userspace)
> - if an operation position is not already in the queue, we add it. If we
> cannot satisfy it, we exit to userspace
> - need to check read-after-write for collision in the queue.
>
> There's some beginning of that already (see struct read_cache).
>
> For now we can live without SIDT to mmio, that means you can do the
> simple fix to writeback() above. But you will only be able to test is
> with realmode.flat, not with emulator.flat since that requires mmio.
>   

If test with realmode.flat, fix to writeback() does not need,
because the size we need to writeback is 2 + 4 in realmode.flat.
I tested used the following patch.

diff --git a/x86/realmode.c b/x86/realmode.c
index 8c771fc..26651a6 100644
--- a/x86/realmode.c
+++ b/x86/realmode.c
@@ -1242,6 +1242,25 @@ void test_lds_lss()
 		outregs.ebx == desc.sel);
 }
 
+
+static struct {
+	u16 limit;
+	unsigned long base;
+} __attribute__((packed)) idt_descr = {
+	0x1234,
+	0x5678,
+};
+
+void test_sidt()
+{	
+	MK_INSN(sidt, "sidt (%eax)\n\t");
+
+	inregs = (struct regs){ .eax = (unsigned long)&idt_descr };
+
+	exec_in_big_real_mode(&insn_sidt);
+	report("sidt", 0, idt_descr.limit == 1023 && idt_descr.base == 0);
+}
+
 void realmode_start(void)
 {
 	test_null();
@@ -1274,6 +1293,7 @@ void realmode_start(void)
 	test_cwd_cdq();
 	test_das();
 	test_lds_lss();
+	test_sidt();
 
 	exit(0);
 }