The SSBS characteristics are:
Allows access to the Speculative Store Bypass Safe bit.
This register is present only when FEAT_SSBS is implemented. Otherwise, direct accesses to SSBS are UNDEFINED.
SSBS is a 64-bit register.
The SSBS bit assignments are:
63 | 62 | 61 | 60 | 59 | 58 | 57 | 56 | 55 | 54 | 53 | 52 | 51 | 50 | 49 | 48 | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 40 | 39 | 38 | 37 | 36 | 35 | 34 | 33 | 32 |
31 | 30 | 29 | 28 | 27 | 26 | 25 | 24 | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
RES0 | |||||||||||||||||||||||||||||||
RES0 | SSBS | RES0 |
Reserved, RES0.
Speculative Store Bypass Safe.
Prohibits speculative loads or stores which might practically allow a cache timing side channel.
A cache timing side channel might be exploited where a load or store uses an address that is derived from a register that is being loaded from memory using a load instruction speculatively read from a memory location. If PSTATE.SSBS is enabled, the address derived from the load instruction might be from earlier in the coherence order than the latest store to that memory location with the same virtual address.
SSBS | Meaning |
---|---|
0b0 |
Hardware is not permitted to load or store speculatively, in a manner that could practically give rise to a cache timing side channel, using an address derived from a register value that has been loaded from memory using a load instruction (L) that speculatively reads an entry from earlier in the coherence order from that location being loaded from than the entry generated by the latest store (S) to that location using the same virtual address as L. |
0b1 |
Hardware is permitted to load or store speculatively, in a manner that could practically give rise to a cache timing side channel, using an address derived from a register value that has been loaded from memory using a load instruction (L) that speculatively reads an entry from earlier in the coherence order fro that location being loaded from than the entry generated by the latest store (S) to that location using the same virtual address as L. |
The value of this bit is set to the value in the SCTLR_ELx.DSSBS field on taking an exception to ELx.
On a Warm reset, this field resets to an IMPLEMENTATION DEFINED value.
Reserved, RES0.
Accesses to this register use the following encodings:
op0 | op1 | CRn | CRm | op2 |
---|---|---|---|---|
0b11 | 0b011 | 0b0100 | 0b0010 | 0b110 |
if PSTATE.EL == EL0 then return Zeros(51):PSTATE.SSBS:Zeros(12); elsif PSTATE.EL == EL1 then return Zeros(51):PSTATE.SSBS:Zeros(12); elsif PSTATE.EL == EL2 then return Zeros(51):PSTATE.SSBS:Zeros(12); elsif PSTATE.EL == EL3 then return Zeros(51):PSTATE.SSBS:Zeros(12);
op0 | op1 | CRn | CRm | op2 |
---|---|---|---|---|
0b11 | 0b011 | 0b0100 | 0b0010 | 0b110 |
if PSTATE.EL == EL0 then PSTATE.SSBS = X[t]<12>; elsif PSTATE.EL == EL1 then PSTATE.SSBS = X[t]<12>; elsif PSTATE.EL == EL2 then PSTATE.SSBS = X[t]<12>; elsif PSTATE.EL == EL3 then PSTATE.SSBS = X[t]<12>;
op0 | op1 | CRn | op2 |
---|---|---|---|
0b00 | 0b011 | 0b0100 | 0b001 |
30/03/2021 20:51; e3551d56dc294a4d55296a6c10544191ada08a8e
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