Re: [PATCH v1 4/4] KVM: MMU: Expose the LA57 feature to VM.
From: Yu Zhang
Date: Mon Aug 21 2017 - 03:50:28 EST
On 8/18/2017 8:50 PM, Paolo Bonzini wrote:
On 18/08/2017 10:28, Yu Zhang wrote:
On 8/17/2017 10:29 PM, Paolo Bonzini wrote:
On 17/08/2017 13:53, Yu Zhang wrote:
On 8/17/2017 7:57 PM, Paolo Bonzini wrote:
On 12/08/2017 15:35, Yu Zhang wrote:
index a98b88a..50107ae 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -694,7 +694,7 @@ static __always_inline int __linearize(struct
x86_emulate_ctxt *ctxt,
switch (mode) {
case X86EMUL_MODE_PROT64:
*linear = la;
- if (is_noncanonical_address(la))
+ if (emul_is_noncanonical_address(la, ctxt))
goto bad;
*max_size = min_t(u64, ~0u, (1ull << 48) - la);
Oops, you missed one here. Probably best to use ctxt_virt_addr_bits
and
then "inline" emul_is_noncanonical_address as "get_canonical(la,
va_bits) != la".
Sorry, I just sent out the v2 patch set without noticing this reply. :-)
The emul_is_noncanonical() is defined in x86.h so that no
ctxt_virt_addr_bits needed in emulate.c, are you
suggesting to use ctx_virt_addr_bits in this file each time before
emul_is_noncanonical_address() is called?
No, only in this instance which uses "48" after the call to
emul_is_noncanonical_address.
Sorry, Paolo. I still do not quite get it.
Do you mean the
*max_size = min_t(u64, ~0u, (1ull << 48) - la);
also need to be changed?
But I do not understand why this statement is used like this. My
understanding is that
for 64 bit scenario, the *max_size is calculated to guarantee la +
*max_size still falls in
the canonical address space.
And if above understanding is correct, I think it should be something
like below:
*max_size = min_t(u64, ~0u - la, (1ull << 48) - la);
The "~0u" part is simply because max_size has 32-bit size (it's an
unsigned int variable), while (1ull << 48) - la has 64-bit size. It
protects from the overflow.
Oh, right. "~0u" is only an unsigned int. Thanks for your clarification. :-)
But what if value of "la" falls in between 0xFFFFFFFFFFFFFFFF and
0xFFFF000000000000?
(1ull << 48) - la may result in something between 0x1000000000001 and
0x2000000000000,
and the *max_size would be 4G - 1 in this scenario.
For instance, when "la" is 0xFFFFFFFFFFFFFFF0(unlikely in practice
though), the *max_size
we are expecting should be 15, instead of 4G - 1.
If above understanding is correct, maybe we should change this code as
below:
@@ -690,16 +690,21 @@ static __always_inline int __linearize(struct
x86_emulate_ctxt *ctxt,
ÂÂÂÂÂÂÂ ulong la;
ÂÂÂÂÂÂÂ u32 lim;
ÂÂÂÂÂÂÂ u16 sel;
+ÂÂÂÂÂÂ u64 canonical_limit;
+ÂÂÂÂÂÂ u8 va_bits;
ÂÂÂÂÂÂÂ la = seg_base(ctxt, addr.seg) + addr.ea;
ÂÂÂÂÂÂÂ *max_size = 0;
ÂÂÂÂÂÂÂ switch (mode) {
ÂÂÂÂÂÂÂ case X86EMUL_MODE_PROT64:
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ *linear = la;
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (emul_is_noncanonical_address(la, ctxt))
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ va_bits = ctxt_virt_addr_bits(ctxt);
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (get_canonical(la, va_bits) != la)
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto bad;
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ *max_size = min_t(u64, ~0u, (1ull << 48) - la);
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ canonical_limit = (la & (1 << va_bits)) ?
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ~0ull : ((1 << va_bits) -1);
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ *max_size = min_t(u64, ~0u, canonical_limit - la + 1);
Does this sound reasonable?
BTW, I did not use min_t(u64, ~0ull - la + 1, (1 << va_bits) - la) here,
because I still would like to
keep *max_size as an unsigned int, and my previous suggestion may cause
the return value of
min_t be truncated.
Yu
And with LA57, may better be changed to:
*max_size = min_t(u64, ~0u - la, (1ull << ctxt_virt_addr_bits(ctxt)) -
la);
And for the above
if (emul_is_noncanonical_address(la, ctxt))
we may just leave it as it is.
Yes, exactly. But since emul_is_noncanonical_address is already using
ctxt_virt_addr_bits(ctxt), it may make sense to compute
ctxt_virt_addr_bits(ctxt) once and then reuse it twice, once in
get_canonical(la, va_bits) != la and once in (1ull << va_bits) - la.
Paolo
Is this understanding correct? Or did I misunderstand your comments? :-)
Thanks
Yu
Paolo